Metamorphic textures and a pressure–temperature (P–T) path
of zoisite eclogite are presented to better understand the metamorphic
evolution of the North-East Greenland eclogite province and this particular
type of eclogite. The eclogite contained the mineral assemblage garnet,
omphacite, kyanite, phengite, quartz and rutile at peak pressure. Partial
melting occurred via breakdown of hydrous phases, paragonite,
phengite and zoisite, based on (1) polymineralic inclusions of albite and
K-feldspar with cusps into host garnet, (2) small euhedral garnet with
straight boundaries against plagioclase, (3) cusps of plagioclase into
surrounding phases (such as garnet), and (4) graphic intergrowth of plagioclase
and amphibole next to anhedral zoisite grains. Isochemical phase equilibrium
modeling of a melt-reintegrated composition, along with
XNa-in-omphacite and Si-in-phengite isopleths, yields a peak pressure
of 2.4±0.1 GPa at 830±30∘C. A peak temperature
of 900±50∘C at 1.9±0.2 GPa is determined using
the rim composition of small euhedral garnet, as predicted by modeling a
crystallized melt pocket. Zoisite growth at the expense of kyanite suggests
that the P–T path crossed the fields of zoisite growth at ∼1.9 GPa, 800–900 ∘C on the modeled phase diagram of the bulk rock. A
point on the exhumation path at ∼1.3 GPa and 750 ∘C is derived from hornblende-plagioclase thermometry and
Al-in-hornblende barometry. The study demonstrates that paragonite, phengite
and zoisite could contribute to partial melting of eclogite at near-peak P
and during exhumation.
Introduction
Partial melting, along with subsequent melt extraction and magma ascent,
leads to geochemical differentiation in large orogens (e.g.,
Clemens, 2006). The lower continental crust contains a significant portion
of mafic rocks (Rudnick and Gao, 2014) and can undergo
partial melting in eclogite facies when incorporated into large continental
orogens. Investigating melting of eclogite from high-pressure (HP) terranes
of large orogens is, therefore, critical to our understanding of geochemical
differentiation processes. Despite retrograde modification of melt-related
textures, evidence of residual melt could be present along grain boundaries
and as inclusions in porphyroblasts (Holness et al., 2011). Minerals
crystallized from melt form low dihedral angles at silicate–melt contacts;
for example, cusps of plagioclase that protrude into surrounding phases
(e.g., omphacite) are interpreted to be crystallized melt (Cao et al.,
2019; Wang et al., 2014). Melt can be captured during mineral growth and
preserved as a crystallized assemblage in peritectic minerals such as
garnet. For example, an inclusion assemblage of K-feldspar + quartz
± plagioclase ± calcite ± barite in garnet and omphacite
in eclogite is interpreted as crystallized from melt that had formed by
breakdown of hydrous minerals (e.g., Gao et al., 2014), such as
phengite, paragonite and zoisite, on a retrograde path (Chen et al., 2014)
or during burial (Massonne, 2011). Clinopyroxene + quartz ± kyanite ± plagioclase inclusions in garnet from an ultrahigh-pressure
(UHP) eclogite from the North-East Greenland Caledonides are interpreted as
crystallized melt formed from breakdown of an epidote-group mineral
(Cao et al., 2019). Graphic intergrowths and peritectic
minerals can also be present in partially melted eclogite. Once the melt
mode reaches a certain threshold (Rosenberg and Handy, 2005), melt
accumulates and channelizes, leading to melt loss from the source rock and
formation of leucocratic layers. Thus, felsic veins, irregular patches and
lenses, with a dacitic (e.g., Wang et al., 2014), granitic
(e.g., Gilotti et al., 2014), tonalitic (e.g., Chen et al.,
2010), and trondhjemitic (e.g., Labrousse et al., 2011)
composition within eclogite bodies, are interpreted as crystallized melt
derived from eclogite.
The North-East Greenland Eclogite Province (NEGEP; Gilotti, 1993),
consisting of quartzofeldspathic gneisses that host eclogite and rare
metapelite, is a large HP terrane that contains a small area of UHP rocks
(Fig. 1; Gilotti and Ravna, 2002). Both eclogite and
metapelite from the UHP terrane were partially melted (Lang and Gilotti,
2007, 2015; Cao et al., 2019). The UHP kyanite eclogite contains plagioclase
cusps, polymineralic inclusions in garnet and omphacite, graphic intergrowths,
and neoblasts, which are interpreted as evidence of partial melting
(Cao et al., 2019). Experimental phase relations (Skjerlie
and Patiño Douce, 2002) suggest that this eclogite was partially melted
by breakdown of an epidote-group mineral on the exhumation path. An
anatectic UHP metapelite contains leucocratic layers and polymineralic
inclusions in garnet that are interpreted as crystallized pockets of melt
derived from phengite breakdown (Lang and Gilotti, 2007).
Gilotti et al. (2004) noted the presence of neosome along with
web-like leucosome in zoisite eclogite (419 655; Fig. 1c) from the HP terrane
near Sanddal and suggested that the eclogite was partially melted. However,
no systematic study has been conducted on partial melting of HP eclogites
from this area.
(a) Index map of the Greenland Caledonides showing the
North-East Greenland Eclogite Province (NEGEP). (b) Geological map of the
southern part of the NEGEP showing localities mentioned in the text. (c) Geological map of the Sanddal area (modified from Hallett et al.,
2014) with the studied outcrop locality marked by a triangle in the
Storstrømmen shear zone. Sample 419655 (triangle) refers to an eclogite
studied by Gilotti et al. (2004). SMUK refers to Sanddal mafic–ultramafic
complex.
In this work, we document textural evidence, infer a partial melting
history and delineate a metamorphic P–T path for zoisite eclogite from the
Sanddal area (Fig. 1) in the NEGEP. Textures, such as plagioclase cusps,
graphic intergrowths and polymineralic inclusions, are interpreted as formed
from crystallized melt. Textures and phase equilibria indicate that
paragonite, phengite and zoisite were the major reactants of the
melt-producing reactions. Phase equilibrium modeling with a
melt-reintegrated composition corroborates the former presence of
paragonite, although it is not preserved in the mineral assemblage. Using
mineral chemistry, textures, isopleth thermobarometry and Zr-in-rutile
thermometry, a P–T path is constructed from peak P of 2.4±0.1 GPa, 830±30∘C to maximum T of 900±50∘C at 1.9±0.2 GPa in suprasolidus, and equilibrium reached at ∼1.3 GPa and 750 ∘C.
Geological setting
The North-East Greenland Eclogite Province covers an area of ∼50 000 km2 of eclogite-facies rocks (Fig. 1; Gilotti, 1993; Gilotti
et al., 2008) on the northeastern margin of Laurentia. The NEGEP was formed in
the overriding plate of Laurentia during the Caledonian continental
collision with Baltica (Gilotti and McClelland, 2007; Gilotti and
McClelland, 2011) and was thrust westward as the structurally highest sheet
north of 76∘ N. The eclogite province is divided into three blocks
– eastern, central, and western – by the NNW-striking Germania Land shear
zone in the east (Hull and Gilotti, 1994; Sartini-Rideout et
al., 2006) and the NNE-striking Storstrømmen shear zone in the west
(Fig. 1b; Holdsworth and Strachan, 1991; Hallett et al., 2014). The NEGEP
is mainly composed of quartzofeldspathic gneiss that contains layers and
lenses of eclogitic rocks. Gneissic protoliths are Paleoproterozoic
granodioritic rocks derived from 2.0 to 1.8 Ga juvenile calcalkaline arcs
that were intruded by anorogenic granites at 1.75 Ga (Kalsbeek et al.,
2008). The HP rocks include eclogite (sensu stricto), garnet clinopyroxenite, garnet
websterite, websterite and coronitic metagabbro, the protoliths of which are
mafic xenoliths in the Paleoproterozoic plutons, layered intrusions and late
mafic dikes (Gilotti, 1993).
Peak metamorphic P–T conditions and ages have been obtained from different
parts of the NEGEP. Kyanite eclogites from Weinschenk Island in the central
block yielded a minimum peak pressure around 1.8 GPa and a peak temperature
of 850 ∘C (Elvevold and Gilotti, 2000). Eclogite and garnet
websterite from the central block point to a peak P–T range of 1.5–1.8 GPa,
750–780 ∘C (Brueckner et al., 1998) using the
garnet-clinopyroxene Fe2+-Mg exchange thermometer
(Ellis and Green, 1979), the albite to jadeite
transformation barometer (Holland, 1980) and the Al-in-orthopyroxene
barometer (Brey and Köhler, 1990; Harley and Green, 1982). Using the
same thermometer and barometers, garnet websterite from the Danmarkshavn
area in the central block returned a peak P–T condition at 2.35 GPa and 790 ∘C (Brueckner et al., 1998). The HP metamorphism is
approximately 420–395 Ma from U-Pb sensitive high-resolution ion microprobe
(SHRIMP) dating of zircon and Sm-Nd mineral isochrons (Gilotti et al.,
2004; Hallett et al., 2014; McClelland et al., 2016). Hallett et
al. (2014) measured trace elements and dated zircon from zoisite eclogite
03-99, adjacent to the studied sample 03-59, with a SHRIMP reverse
geometry instrument. The zircon has a low U and Th/U metamorphic rim around
a higher U and Th/U core. Zircon cores of 2010±10 Ma give the age of
the protolith. The rim shows a flat HREE (heavy rare earth elements) pattern and no Eu anomaly, typical
of zircon crystallized in garnet-bearing eclogite-facies rocks (Rubatto,
2002), and yielded an age of 394±12 Ma, which is interpreted as the
age of HP metamorphism.
An UHP terrane has been identified in the easternmost part of the hinterland
on a small island at 78∘ N (Gilotti and Ravna, 2002).
The regional extent of the UHP terrane is unknown, but it does not comprise
the entire eastern block because HP eclogites are documented in Holm Land
(McClelland et al., 2016). The UHP metamorphism is confirmed by coesite
inclusions in zircon (McClelland et al., 2006). Peak P–T conditions for the UHP
terrane were estimated to be 3.6 GPa, 970 ∘C by empirical
thermobarometry (Gilotti and Ravna, 2002) and slightly lower at 3.4 GPa,
940 ∘C using phase equilibrium modeling (Cao et
al., 2019). The UHP metamorphism is 365–350 Ma or approximately 50 million years
younger than the HP metamorphism (McClelland et al., 2006; Gilotti et
al., 2014). U-Pb dating of later zircon domains reveals that partial melting
started at 347 Ma and lasted until ≈320 Ma (Gilotti et al., 2014).
Outcrop and petrographic description
The Sanddal area is part of the western block of the NEGEP, which extends
from the Storstrømmen shear zone (SSZ) to the inland ice. The SSZ is
mainly composed of protomylonites to ultramylonites, with an exposed
thickness of <1 km (Hallett et al., 2014; Hull and Gilotti,
1994). The area exposes mainly quartzofeldspathic gneisses with eclogitic
pods, metagranite, orthogneiss, rare paragneiss and pegmatite
(Hallett et al., 2014). Partially eclogitized, coronitic
gabbro-norite pods occur in the Sanddal mafic–ultramafic complex (SMUK;
Lang and Gilotti, 2001), which is located ∼ 5 km west of the
SSZ (Fig. 1c).
The studied outcrop lies within the SSZ (UTM coordinates are zone 27× VG 492 581, 8 657 828 m); Gilotti collected the samples in 2003. A 10 m long pod
with well-preserved eclogite enclosed in quartzofeldspathic protomylonite
(Fig. 2a–d) was selected for this study. The eclogitic pod is heterogeneous
(Fig. 2a–d). The inner part is massive and mainly consists of garnet-rich
restite, with bimineralic eclogite, quartz eclogite and zoisite eclogite,
whereas outer parts display a strong gneissosity to mylonitic foliation in
dominantly zoisite-rich gneiss. We focus on two samples, 03-57 and 03-59,
that are remarkably well preserved in a low-strain boudin, despite their
location in a shear zone; the two samples are representative of the inner
and outer parts of the eclogite pod, respectively. Leucocratic veins, both
concordant and discordant and up to 5 cm thick, in the eclogite pods (Fig. 2a–d) are mainly composed of plagioclase, quartz and amphibole. A
leucotonalitic orthogneiss is located just west of the pod and extends
discontinuously for 3 km along the SSZ (Fig. 1c).
Photographs of the studied outcrop from the
Storstrømmen shear zone near Sanddal. The orange color is lichen; pencil
for scale. (a) Eclogite pod with leucocratic veinlets surrounded by foliated
zoisite eclogite and quartzofeldspathic gneiss. (b) Felsic veinlets in the
center of the eclogite pod. (c) Eclogite pod with felsic veins surrounded by
foliated zoisite eclogite. The white box marks the position of (d). (d) Close-up of
internally derived felsic vein in eclogite and connection with felsic
zoisite eclogite.
Modes were determined using PetrogLite™ point-counting software with
an automated Petrog™ stepping stage at the Department of Earth and
Environmental Sciences, University of Iowa. A total of 200 points were counted
per thin section. Metamorphic assemblages and melt-related textures are
described for the two eclogite samples and then used for deciphering the
metamorphic evolution.
Zoisite eclogite 03-57
The sample is a partially retrogressed zoisite eclogite collected from the
inner part of the mafic pod. Mineral modes (in vol. %) are 11 % garnet,
16 % omphacite and diopside, 29 % amphibole, 18 % plagioclase, 4 %
kyanite, 14 % diopside-plagioclase-amphibole symplectite, 5 % quartz,
3 % zoisite, and accessory biotite and rutile. The eclogite is weakly
foliated with millimeter-thick plagioclase and quartz layers comprising 10 %–15 %
of the whole rock. Figures 3a–d and 4 show the representative textures in
the eclogite.
Photomicrographs of representative textures: (a–d) are
from sample 03-57 and (e–f) are from 03-59. Red arrows mark interstitial
plagioclase into garnet–garnet or garnet–zoisite boundary. Yellow arrows
point to euhedral garnet against plagioclase. Black arrows mark embayed
boundaries of clinopyroxene or zoisite. (a) Small euhedral garnet (Grt II)
growing on large garnet (Grt I), both show a euhedral face against
plagioclase. Garnet grains are adjacent to subhedral zoisite surrounding
kyanite and anhedral diopside (cross-polarized light, XPL). (b) Graphic
intergrowth of amphibole and plagioclase next to anhedral diopside and
kyanite, which is surrounded by anhedral zoisite. Note that the amphibole
grains have the same crystallographic orientation (XPL). (c) Kyanite
surrounded by anhedral zoisite, which is next to poikiloblastic plagioclase.
The plagioclase displays cusps into diopside and zoisite (plane-polarized
light, PPL). (d) Backscattered electron (BSE) image showing polymineralic
inclusions in plagioclase adjacent to Grt I with euhedral crystal faces.
The polymineralic inclusions of kyanite + K-feldspar + albite + Grt II show sharp offshoots into plagioclase. (e) Euhedral Grt II,
zoisite, omphacite and plagioclase. Plagioclase shows cusps into Grt
II–zoisite boundaries (XPL). (f) Photomicrograph showing Grt II with a euhedral
crystal face against plagioclase adjacent to zoisite. Inclusions are common
in the core and rare in the rim.
BSE images of inclusions in eclogite 03-57. (a) Porphyroblastic garnet (Grt I) with abundant polymineralic inclusions and
fractures. The black box shows the position of (b). (b) Polymineralic
inclusions of albite and K-feldspar along with cracks that are filled with
the same mineral assemblage. (c) Polymineralic inclusions of K-feldspar and
kyanite in Grt I. (d) Garnet I contains numerous poly- and monomineralic
inclusions. The black box shows the position of (e). (e) Polymineralic
inclusions of albite, apatite and phengite. The five equally spaced bright
spots (blue arrow) on the polymineralic inclusions are caused by a line scan
of garnet (Grt I) during analysis. (f) Polymineralic inclusion of albite
and K-feldspar in Grt I. Note the cuspate contact marked by the red
arrow.
Two generations of garnet, larger (Grt I) and smaller (Grt II), are present.
Large garnet (Grt I), up to centimeter sized, is anhedral with embayed grain
boundaries, and rarely displays straight boundaries against plagioclase
(Figs. 3a and 5). Abundant inclusions are present in the core of Grt I, with
fewer inclusions in the rim. The inclusions are monomineralic omphacite,
kyanite, plagioclase, K-feldspar, albite, zoisite, quartz and rutile, as well as
polymineralic inclusions (Fig. 4a–d). Polymineralic inclusions are dominated
by K-feldspar ± albite ± kyanite ± phengite ± apatite (Fig. 4a, b), while polymineralic inclusions of phengite +
kyanite, K-feldspar + allanite and K-feldspar + albite + zoisite are
present in the inner rim. The K-feldspar + albite inclusions display cusps
into each other (Fig. 4b, f) and into Grt I (Fig. 4e, f). Fractures in the
large garnet are filled by the same mineral assemblage as the connected
polymineralic inclusions (Fig. 4b). Small euhedral Grt II, about 200 µm in diameter, contains few inclusions and straight boundaries against
plagioclase (Fig. 3a).
X-ray maps showing compositional zoning of garnet (a–d) and phengite (e–f) in zoisite-eclogite 03-57. The garnet displays a
relatively homogeneous core, which contains numerous inclusions and a zoned
inner rim and outer rim. The inner rim of the garnet contains few inclusions and
displays patchy zoning in Mg and Ca. The garnet outermost rims are richer in
Fe, Ca and Mn. (e–f) Phengite shows slight variation of Mg and thus Si and Ti.
The matrix is mainly composed of clinopyroxene, kyanite, amphibole, zoisite,
biotite, plagioclase and K-feldspar. Matrix kyanite, subhedral in shape, is
commonly rimmed by subhedral to anhedral zoisite (Fig. 4a). Phengite is only
present as inclusions in garnet (Fig. 5), but minor biotite clusters in the
matrix indicate the former presence of phengite at peak pressure. Diopside
plus plagioclase and amphibole forms symplectite around omphacite. The width
of the symplectite lamellae varies from 5 to 50 µm (Fig. 3c). Amphibole
is anhedral or poikiloblastic. The anhedral amphibole is blocky and large
(up to 800 µm), replacing Grt I with plagioclase. The poikiloblastic
amphibole is intergrown with plagioclase, which displays cusps pointing into
amphibole. Graphic intergrowth of amphibole and plagioclase is observed
locally in the matrix, adjacent to anhedral zoisite (Fig. 3b–c), forming
millimeter-long channel-like features. Zoisite surrounds subhedral to anhedral up to
millimeter-sized kyanite and is anhedral to subhedral adjacent to plagioclase (Fig. 3a–c). Plagioclase occurs in the symplectite and the matrix and forms cusps
into anhedral clinopyroxene (Fig. 3a–c). Matrix plagioclase (Fig. 3d)
adjacent to an euhedral crystal face of Grt I contains a polymineralic
inclusion of garnet + kyanite + K-feldspar + albite. Kyanite in the
inclusion is rimmed by K-feldspar and in turn surrounded by albite; the albite
displays sharp offshoots into the host plagioclase. Cusps of plagioclase
into clinopyroxene, amphibole, zoisite and garnet–garnet boundaries are also
observed in the matrix (Fig. 3a–c). Rare brown biotite, either in the matrix
or enclosed in zoisite, coexists with plagioclase, kyanite and Grt II.
These textures suggest that the peak pressure assemblage is Grt I,
omphacite, kyanite, phengite, quartz and rutile (Figs. 3 and 4; Table 2), while
the lower-pressure assemblage contains Grt II and polymineralic inclusions
(e.g., with the presence of K-feldspar and/or albite) in the core and inner rim
of Grt I, diopside, kyanite, zoisite, amphibole, plagioclase and minor
biotite (Fig. 3a–d; Table 2). Polymineralic inclusions and
fractures within garnet suggest that the inclusions are crystallized melt.
Assemblage and textural settings of minerals from the
studied samples. Abbreviations for the textural relation are as follows: pb, porphyroblast;
pbt, poikiloblast; m, matrix; i, inclusion; s, symplectite; r, retrograde
mineral; a, accessory. Abbreviations for minerals are as follows: Cpx, clinopyroxene
including omphacite and diopside; Amp, amphibole; Ph, phengite; other
mineral abbreviations follow Kretz (1983).
Developmental sequence of the metamorphic minerals in
matrix of the studied samples: phengite and zoisite in the matrix and
phengite and paragonite in garnet broke down during partial melting. The dashed line indicates inferred presence of mineral.
Zoisite eclogite 03-59
Sample 03-59 is a zoisite eclogite collected from the foliated margin of the
eclogite pod. A strong foliation is defined by millimeter-thick leucocratic layers
alternating with zoisite-rich layers. The peak mineral assemblage is similar
to that in sample 03-57, i.e., garnet, omphacite, kyanite, zoisite,
phengite, quartz and rutile. A distinctive feature is the very high
abundance of zoisite (36 %) oriented sub-parallel to the major foliation.
Zoisite occurs as euhedral to subhedral crystals up to 1 cm long (Fig. 3e).
Textures of the peak assemblage differ from sample 03-57: Grt II (9 %) is
small (<1 mm), euhedral to subhedral (Fig. 3e), with polymineralic
inclusions of plagioclase, kyanite, quartz and phengite (?) common in the
core and rare in the rim (Fig. 3f). Matrix kyanite (7 %) is subhedral and
commonly enclosed by zoisite and plagioclase. Subhedral to anhedral
clinopyroxene (8 %) in the matrix typically shows sharp embayments by
plagioclase. Accessory rutile and zircon are present as inclusions and
matrix minerals. The sample is strongly laminated with leucocratic layers of
various thickness (up to 5 mm) composed of plagioclase, large quartz grains,
minor small euhedral garnet and anhedral clinopyroxene. Omphacite is
partially replaced by symplectitic intergrowths of diopside and plagioclase
with or without amphibole. Diopside rims adjacent to the leucosomes
typically show sharp embayments by plagioclase or quartz. Poikiloblastic
plagioclase (17 %) forms cusps into subhedral to anhedral amphibole
(15 %) and Grt II-zoisite (Fig. 3e, f) and typically surrounds anhedral
kyanite. Minor biotite (∼1 %) coexists with plagioclase in
the leucocratic layers.
Mineral chemistryAnalytical conditions
Major element composition of minerals was determined using a Cameca SX-100
electron microprobe (EMP) equipped with five wavelength-dispersive
spectrometers at the Institut für Mineralogie und Kristallchemie,
Universität Stuttgart, Germany. Natural albite, orthoclase, diopside, and
rhodonite and synthetic hematite, periclase, BaSO4, TiO2,
Cr2O3, and Al2O3 were used as standards. X-ray maps of
garnet were obtained with operational conditions of 15 kV accelerating
voltage, 100 nA beam current and 70 ms pixel time with step sizes from 6 to
8 µm. Phengite was mapped using 15 kV accelerating voltage and 30 nA
beam current with 3 µm step size. Points were analyzed with 15 kV
accelerating voltage for all minerals, and 15 nA beam current for garnet and
kyanite and 10 nA for all other minerals except rutile. Counting time was 20 s for peak and background analysis.
Point analysis of rutile was performed with 15 kV accelerating voltage and
90 nA beam current with a focused beam. To monitor the possible influence of
surrounding silicate minerals, Si was also measured. The peak and background
counting times for Zr, Cr, Fe, Nb and Si were set at 100 s. Wollastonite,
J43_Zir, Cr2O3, Fe2O3 and Nb were used as
standards.
The PAP correction procedure by Cameca was used to correct raw counts.
Chemical composition was calculated with Excel spreadsheet CALCMIN
(Brandelik, 2009) for all minerals except amphibole for which the Excel
spreadsheet by Locock (2014) was used, following the nomenclature of
Hawthorne et al. (2012). Ferric iron (Fe3+) is calculated for all
minerals. Calculated mineral formulae are presented in Tables 3–6.
Representative analyses of garnet from the samples 03-57
and 03-59. Vanadium was analyzed but its concentration is below the
detection limit of the microprobe, and thus is neglected from the data. Garnet is normalized to 24 oxygen, with 10 six- and eight-fold
coordinated cations. Ferric iron (Fe3+) is calculated based on a total
of four atoms on the octahedrally coordinated site.
All
iron is treated as ferrous iron in the analytical results, as marked by the
star (∗).
Representative analyses of clinopyroxene in the zoisite
eclogites 03-57 and 03-59. Ferric iron is calculated by charge balance,
while Al_T and Al_O stand for aluminum in
tetrahedral and octahedral sites, respectively. Clinopyroxene is normalized
to six oxygen and four cations.
Representative analyses of matrix minerals in the zoisite
eclogite. The dash (-) refers to unanalyzed data, while b.d. refers to
below detection limit. Ferric iron (Fe3+) in amphibole is calculated
based on charge balance (see Locock, 2014, for details). All iron in
feldspar and zoisite is assumed to be ferric. XFe3+ is defined as 100∗Fe3+/(Fe3++Al). Feldspar is normalized to 8 oxygen, zoisite to
12.5 oxygen, and amphibole to 46 valences.
Representative analyses of minerals in garnet in the
zoisite eclogite 03-57. Ph1 is the pristine phengite (Fig. 6) and
Ph2 occurs as a mineral in polymineralic inclusion in garnet. The
dash (-) marks unanalyzed elements, while b.d. marks those below
detection limit. Structural formulae for minerals other than phengite were
calculated as given in Tables 4 and 5. Phengite is recalculated based on 42
valences for the four- and six-fold cations.
Garnet I displays slight and patchy zoning (Fig. 5a–d; Table 3). The core
is composed of 40 mol % almandine, 34 mol % pyrope, 24 mol % grossular
(+andradite) and 1 mol % spessartine
(Alm40Prp34Grs24Sps1). Almandine increases from core to
rim to Alm44, pyrope increases to Prp36 at the inner rim and
decreases to Prp31 at the outermost rim, and grossular decreases to
Grs21 at the inner rim and increases to Grs22 at the outermost rim
(Fig. 6a; Table 3). Spessartine is relatively constant at Sps1 and
increases to Sps2 at the outermost rim. Grt II has similar composition
to the rim of Grt I.
Diagrams showing compositional variation of garnet,
clinopyroxene, amphibole and phengite. (a) Compositional zoning of garnet
along the profile shown in Fig. 5a. Primary axis on the left is for Alm, Prp
and Grs, whereas the secondary axis on the right is for Sps. (b) Ternary diagram
showing composition of clinopyroxene varying from omphacite to diopside.
Black circles are for analyzed mineral in 03-57, and blue triangles are for those in
03-59. (c)M4(Na + K + 2Ca) vs. M1(Al + Fe3++ Ti)
diagram showing amphibole chemistry, nomenclature follows Hawthorne et al. (2012); symbols are the same as (b). (d) Mg vs. Si p.f.u. diagram showing the
composition of phengite in sample 03-57.
Clinopyroxene of variable composition is present as inclusions in garnet, in
the matrix and in symplectite (Fig. 6b; Table 4). Inclusions in Grt I are
omphacite, with XNa [= Na/(Na + Ca)] up to 0.40. Matrix
clinopyroxene varies from omphacite to diopside with a Na-rich core
(XNa up to 0.43–0.44) and a Na-poor rim (XNa=0.16).
Clinopyroxene in the symplectite is diopside.
All amphibole is calcic, specifically pargasite and sadanagaite (Fig. 6c;
Table 5). Ferric iron, calculated using the charge balance method
(Locock, 2014; Hawthorne et al., 2012), is <0.40 p.f.u (atoms per formula unit). Zoisite
(Table 5) in garnet contains Fe3+ between 0.12 and 0.14 p.f.u., whereas
matrix zoisite shows a slightly larger range, 0.10–0.14. No systematic
zoning is seen in zoisite.
Feldspar includes both K-feldspar and plagioclase (Fig. 3; Tables 5, 6). The
plagioclase inclusions in garnet are nearly pure albite to oligoclase
(XAn: 0.16–0.28). The matrix plagioclase is also oligoclase (XAn:
0.24–0.30). Plagioclase in symplectite is oligoclase and andesine
(XAn: 0.28–0.33).
The pristine phengite inclusion in Grt I contains Si between 3.15 and 3.27 p.f.u. (Figs. 5e–f and 6d; Table 6). One analyzed phengite in a
polymineralic inclusion in the rim of Grt I yielded a total of
∼95.6 wt % and a Si content of 3.41 p.f.u. (Table 6).
Kyanite contains a small amount of Fe3+, up to 0.08 p.f.u. Rutile
enclosed in garnet contains 470–840 ppm Zr, whereas the Zr content in
matrix rutile ranges between 550 and 630 ppm.
Zoisite eclogite 03-59
Garnet is slightly zoned, varying from Prp34Alm41Grs24 at the
core to an inner rim of Prp38Alm41Grs22; the composition
changes to Prp33Alm41Grs24 at the outermost rim. Spessartine
is constant at ∼1 mol %. Matrix clinopyroxene is omphacite
(XNa up to 0.43) in the core and diopside (XNa=0.14) in the
rim (Table 4). Clinopyroxene in the symplectite is diopside (Fig. 6b).
Feldspar inclusions in clinopyroxene are oligoclase (XAn≈0.27; Table 5). The matrix feldspar is also oligoclase (XAn=0.24–0.30, Xkfs<0.01); the plagioclase displays a slight
zoning with a lower anorthite core (XAn=0.24) and higher anorthite
rim (XAn=0.29). Symplectitic plagioclase coexisting with biotite in
pseudomorphs after phengite has XAn=0.27. Plagioclase in
symplectite after omphacite shows XAn=0.28–0.31.
Amphibole is calcic, specifically pargasite and tschermakite (Fig. 6c; Table 5). Zoisite is slightly zoned with Fe3+ varying from 0.09 to 0.15 p.f.u. (all iron is assumed to be Fe3+). Kyanite inclusions in garnet
contain up to 0.17 p.f.u. Fe3+. Phengite in a polymineralic inclusion
in garnet shows a total of ∼94.7 wt % with a normalized
composition with Si = 3.48 p.f.u. Matrix rutile contains 690 to 890 ppm Zr.
Isochemical phase equilibrium modeling was conducted using the Gibbs free energy minimization method included in the software package
PERPLE_X ver. 6.7.7 (Connolly, 2005). Three P–T phase
diagrams were constructed for the sample 03-57 according to the following
strategy. (i) The first pseudosection was modeled using the measured bulk
rock composition with the 11-component system MnNCKFMASHTO
(MnO-Na2O-CaO-K2O-FeO-MgO-Al2O3
-SiO2-H2O-TiO2-O2).
A small amount of oxygen, as a proxy for Fe2O3 in the bulk rock,
and a free fluid phase of pure H2O were added. This pseudosection was
used to infer the initial melt composition to be added back to the measured
bulk composition in order to obtain a melt-reintegrated bulk composition and
to infer the retrograde evolution after the attainment of peak T. (ii) The
second diagram was calculated in a reduced 8-component system MnNCFMASH to
model a garnet-bearing melt-pocket (Fig. 3a). Microstructural evidence
suggests that the studied samples were partially melted: the polymineralic
inclusions in garnet and graphic amphibole + plagioclase (Fig. 3b) and
plagioclase + diopside + Grt II in the matrix (Fig. 3a) represent former
melt pockets. Accordingly, this pseudosection was used to constrain peak T. (iii) An additional melt-reintegrated pseudosection was modeled
with the MnNCKFMASHTO system to reconstruct the P–T history before melt loss
(i.e., before peakP), by adding 10 % melt back to the measured bulk
composition. The bulk composition changes if melt is lost from the system
(Yakymchuk and Brown, 2014), particularly after reaching a threshold of 7 vol % (Rosenberg and Handy, 2005). Models can address this issue by
integrating the estimated composition of lost melt back into the measured
bulk composition (e.g., Bartoli, 2017, and references therein).
The internally consistent thermodynamic dataset of Holland and Powell (2011) for minerals and H2O (the CORK model; Holland and Powell,
1991) was used, together with solution models that are compatible with this
dataset. The models include Omph(GHP) for Na-poor clinopyroxene and
omphacite, cAmph(G) for clinoamphibole, melt (G) for melt (Green et al., 2016), Gt(W) for
garnet, Mica(W) for potassic white mica (e.g., phengite) and paragonite,
Bi(W) for biotite, Chl(W) for chlorite (White et al., 2014), T for talc
(Holland and Powell, 2011), and feldspar for both plagioclase and
K-feldspar (Fuhrman and Lindsley, 1988). Garnet contains a small
amount of spessartine, and thus this end-member was restricted to 10 mol % in
the solution model. Lawsonite, quartz, zoisite and kyanite were treated as
pure phases. The applied melt model was originally developed for P<1.3 GPa but has been extrapolated to 2.6 GPa with geologically meaningful
results (Wade et al., 2017). The compositional ranges of the
amphibole and melt models were limited iteratively during the calculations.
The keyword values for initial_resolution,
final_resolution, iteration value 2 and
refinement_points_II in the
perplex_option.data file were set at 0.100, 5e-3, 1 and 1,
respectively, minimizing the number of pseudocompounds during calculation.
Modeling resultsPseudosection for the measured bulk rock composition
The composition of the bulk rock for sample 03-57 was acquired using
wavelength-dispersive X-ray fluorescence (XRF) spectrometry at Washington
State University. The analyzed bulk composition was adjusted for
PERPLE_X in the chosen systems (Table 7). CaO was reduced by
the amount of corresponding P2O5, assuming that all phosphorus is
bound to ideal apatite. The initial water used is 0.75 wt %, estimated
using the water content (Schmidt and Poli, 2014), density and
proportion of hydrous minerals. Since a small amount of ferric iron is
present in omphacite, kyanite, zoisite and amphibole, the oxygen content was
estimated corresponding to 10 % of total iron being trivalent. The total
components were normalized to 100 %.
Whole rock bulk composition determined by X-ray
fluorescence (XRF), along with adjusted bulk composition; domainal
composition of a melt pocket; reintegrated melt composition at 2.2 GPa, 802
∘C; and melt-reintegrated composition for sample 03-57 that are
used for modeling. All are given in weight percentages (wt %). The
amount of O2 for the adjusted bulk rock is estimated to be equivalent
to 10 % Fe3+ of total iron. The dash (–) marks the
elements not determined or considered.
The P–T pseudosection was calculated for the range of 1.0–2.5 GPa and 600–950 ∘C (Fig. 7 and Fig. S1 in the Supplement). Garnet and clinopyroxene
are ubiquitous in the modeled phase diagram. The solidus curve shows a
positive dP/ dT slope at P>1.95 GPa and a negative slope at P<1.4 GPa (Fig. 7). Melt is only present at T>750∘C;
the highest amount of melt predicted within the P–T range is 12 % at 2.0 GPa, 900 ∘C. This pseudosection cannot be used to retrieve P–T
conditions before melt loss.
P–T pseudosection of sample 03-57 with an XRF-derived bulk
composition, showing the mineral assemblages and highlighted phase
boundaries of kyanite, zoisite, melt (L). Abbreviations are in Table 1; V
= free water. Dashed line of NaCpx indicates Na content in M2 site in
clinopyroxene; the line separates the mineral assemblages with calcic
pyroxene (augite, NaCpx< 0.20) from those with sodic–calcic
pyroxene (omphacite, NaCpx> 0.20). The purple area marks
where the highest values of Si-in-Ph and XNa-in-Cpx intersect. This
area provides an estimate of peak P to use in determining a P–T path across which
an initial melt that is reintegrated into bulk composition is generated
(marked by the blue star). Zoisite modes at 1.7–1.9 GPa, 800–880 ∘C are also marked.
To estimate the approximate prograde path prior to melt loss, a certain
amount of melt needs to be reintegrated into the measured bulk composition.
The melt reintegrated pseudosection is used to estimate a prograde path
across which an initial melt was generated. In this case, the estimated peak
conditions are not the true peak and are used solely for deriving a P–T path
that crosses the solidus, where initial melt composition (Table 7) is
determined for modeling a melt re-integrated pseudosection. Using isopleths
of Si-in-phengite and XNa-in-clinopyroxene, the phengite with average
high Si content (3.24–3.27 p.f.u.) and the omphacite with average high
XNa value (0.43–0.44) intersect at 2.3±0.2 GPa, 770±60∘C. The composition of the initial melt to be reintegrated into
the measured bulk composition was granitic senso lato and determined at P=2.2 GPa and
T=800∘C.
The pseudosection modeled with the measured bulk rock composition yields
additional P–T conditions, based on the observed zoisite-bearing textures.
Zoisite overgrowths on kyanite (e.g., Fig. 3b, c) indicate that the rock
followed a path that leads to increasing modes of zoisite and decreasing
modes of kyanite. The pseudosection predicts that, at 800–900 ∘C, the kyanite-out and the zoisite-in boundaries are nearly parallel and
located close to each other, thus defining narrow fields containing both
phases at 1.9–2.1 GPa. These Ky + Zo-bearing fields represent a nearly
discontinuous reaction that consumes kyanite and produces zoisite at
decreasing pressure. The observed kyanite–zoisite relationship suggests that
the rock followed a P–T path that crossed this reaction. Moreover, the anhedral
shape of zoisite adjacent to leucocratic layers of crystallized melt
suggests that the sample traversed through areas of zoisite-melting located
at P=1.7–2.0 GPa and T=800–900 ∘C (Fig. 7).
Pseudosection for a crystallized melt pocket
A crystallized melt pocket (Fig. 3a) was modeled to determine the
equilibrium conditions for melting (i.e., peak-T conditions). Bulk composition
(Table 7) for the crystallized melt pocket (Figs. 3a, 8) was calculated
with estimated modes of 35 % peritectic garnet, 55 % plagioclase and
10 % clinopyroxene along with their corresponding mineral composition.
Phase densities used are 2.64 g cm-3 for plagioclase (Phillipsi et
al., 1971), 3.29 g cm-3 for clinopyroxene (Mottana et al.,
1979) and 3.7 g cm-3 for garnet (Nestola et al., 2012) because
the experimental mineral composition is similar to those in the crystallized
melt pocket. An arbitrary value of 1 wt % water was used. The
pseudosection calculated with this estimated bulk composition shows that the
assemblage of garnet, plagioclase, omphacite and melt is located at 1.9–2.2 GPa and 850–950 ∘C within the modeled P–T range (Figs. 8, S2). The isopleths of XMg-in-garnet, XFe-in-garnet
and XNa-in-plagioclase do not intersect at a point, but in an area of
1.9±0.2 GPa, 900±50∘C, which is interpreted as
the approximate peak T condition.
P–T pseudosection modeled for a melt domain in zoisite
eclogite 03-57; the solidus curve is highlighted. The field with the
observed mineral assemblage of garnet, plagioclase, omphacite and melt are
shown with bold fonts at 1.9–2.1 GPa, 850–950 ∘C. The dashed line
of NaCpx and abbreviations are the same as in Fig. 7.
Pseudosection for melt-reintegrated composition
The petrographic textures (e.g., thin leucocratic layers interpreted as melt
channels) in sample 03-57 suggest a small amount of melt loss. An accurate
estimate of melt loss is difficult to determine (e.g., Indares et al., 2008),
but a value of 10 % is used, based on the proportion of leucocratic layers
in the sample. The melt-reintegrated pseudosection returns the solidus to
water-saturated conditions (Groppo et al., 2012; Indares et al., 2008).
The bulk composition (Table 7) used to calculate this pseudosection (Figs. 9, S3) reintegrated 10 vol % of melt into the measured
bulk rock using a single-step approach (e.g., Indares et al., 2008;
Bartoli, 2017). The melt composition (Table 7) that was added back into the
measured bulk rock is derived from the first pseudosection at the potential
peak P (see Sect. 5.2.1). This represents the first melt produced by the
source. Since the suprasolidus topology does not significantly change in
response to bulk composition due to melt loss (White and Powell, 2002),
reintegration of the escaped melt with the residual rock would approach the
original rock composition.
Calculated P–T phase diagram modeled with a
melt-reintegrated composition of sample 03-57. The pseudosection is
highlighted for the phase boundaries of paragonite (Pg) and melt, and is
saturated with fluid at subsolidus conditions. Isopleths of Si-in-Ph and
XNa-in-Cpx are plotted on the modeled diagram, with the ellipse
indicating where determined compositions intersect at peak P. The dashed line of
NaCpx and mineral abbreviations are the same as in Fig. 7.
The melt-reintegrated pseudosection shows that the observed peak mineral
assemblage of garnet, omphacite, phengite, kyanite, quartz and rutile is
predicted to be stable at 2.1–2.5 GPa, 720–850 ∘C (Fig. 9),
which is located in a similar P–T range as predicted by the pseudosection
modeled with the measured bulk rock composition (Table 7; Fig. 7). Isopleths
of phengite included in garnet (Si = 3.24–3.27) and omphacite (XNa=0.43–0.44) yielded a peak P of 2.4±0.1 GPa at 830±30∘C (Fig. 9). Additionally, paragonite is predicted to have been
present at P=1.3–2.0 GPa and T=600–730 ∘C. To form
the melting textures with Na- and K-rich phases (e.g., albite and K-feldspar)
in garnet, Na- and K-rich sources are needed; paragonite and phengite
predicted by the pseudosection are the best candidates. Therefore, the
prograde path of the zoisite eclogite is interpreted to have passed through
the paragonite- and phengite-present fields.
Integrating results from the three pseudosections, eclogite 03-57 displays a
clockwise P–T path (Fig. 10). On the prograde path, paragonite formed in the
P–T range of 1.3–2.0 GPa, 600–730 ∘C and phengite was present in
the sample, while garnet grew and included both minerals (Fig. 9). The
sample reached peak P of 2.4±0.1 GPa at 830±30∘C.
The mineral assemblage of garnet + omphacite + kyanite + zoisite +
phengite + quartz + rutile ± melt formed at these P–T conditions
(Fig. 7). With subsequent exhumation and increase in T, partial melting
occurred both in the matrix and in garnet (Figs. 3, 4) resulting in the
formation of new peritectic garnet (Fig. 3a). The pseudosection modeled for
a crystallized melt pocket suggests that a peak T of 900±50∘C was attained at 1.9±0.2 GPa (Fig. 8).
P–T phase diagram showing the delineated P–T path (dashed light blue
curves) for the studied eclogites along with modeled and experimental
phase relations. A to D mark the determined P–T stages, with the polygons,
ellipse and star referring to determined P–T conditions. A marks
the P–T field where paragonite is present. B is determined by the intersection
of Si-in-phengite and XNa-in-Cpx. C refers to the P–T conditions where
zoisite grew at the expense of kyanite and the stable assemblage of
the modeled melt pocket. The star at D is determined using empirical
amphibole-plagioclase thermometry and Al-in-hornblende barometry. Phase
boundaries of solidus (black), kyanite and zoisite from pseudosection of
measured bulk composition, and solidus (dark green) from melt-reintegrated
pseudosection are shown by solid lines. The theoretical phase boundary of
zoisite by Vielzeuf and Schmidt (2001) (VS01) is the dotted line. Long
dashed lines marked as L09 represent phase boundaries of melt and phengite
in experiments by Liu et al. (2009) on phengite-zoisite
eclogite, whereas dot–dashed lines (SPD02) are from Skjerlie and
Patiño Douce (2002) on zoisite eclogites, and short dashed lines (LC01)
are from Lopez and Castro (2001) on amphibolite.
Empirical thermobarometry and trace element thermometryEmpirical thermobarometry
Empirical garnet-clinopyroxene-phengite-kyanite-SiO2 thermobarometry
(Ravna and Terry, 2004) was used to estimate equilibrium conditions for
sample 03-57. Omphacite with the highest jadeite content (XNa=0.43–0.44), pristine phengite (Si p.f.u.: 3.24–3.27) and garnet with the
highest XPrp value (Alm43Prp37Grs19) return an
equilibrium P–T condition of 2.57 GPa, 818 ∘C. Uncertainties in the
original garnet-phengite-clinopyroxene-kyanite-quartz thermobarometry are
±0.32 GPa and ±65∘C (Ravna and Terry,
2004). Pressure calculated by empirical thermobarometry is within the range
of error of the P–T results from the pseudosection (2.4±0.1 GPa, 830±30∘C).
Hornblende-plagioclase thermometry by Holland and Blundy (1994) and
Al-in-hornblende barometry (Anderson and Smith, 1995) are used
iteratively to retrieve exhumation conditions. The crystallized melt pocket
does not contain quartz, hence we used the calibration based on the
net-transfer reaction edenite + albite = richterite + anorthite.
Uncertainty for the amphibole-plagioclase thermometry is given as ±40∘C with potential larger errors for Fe-rich amphibole (Holland
and Blundy, 1994), and for the Al-in-hornblende barometry it is 0.06 GPa
(Anderson and Smith, 1995). The calculation with oligoclase (XAb=0.71) and sadanagaitic amphibole (Table 5) yielded crystallization at
∼1.3 GPa, 750 ∘C.
Zr-in-rutile thermometry
Zirconium incorporation in rutile, in equilibrium with quartz and zircon,
has been developed into an empirical single mineral thermometer (Zack et
al., 2004) and experimentally calibrated to incorporate the pressure effect
(Tomkins et al., 2007; Watson et al., 2006). The studied eclogites
contain zircon and quartz, which renders the thermometry suitable to obtain
crystallization temperatures. Conditions determined by pseudosections fall
within the α-quartz field, thus the Zr-in-rutile thermometry for
this field is used (Tomkins et al., 2007). At peak P of 2.4 GPa, rutile
inclusions in garnet in sample 03-57 yielded a T range of 723–777 ∘C, while matrix rutile gave T=737–749 ∘C (Tomkins et al.,
2007). At P=1.3 GPa, the thermometer returns a T at 674–726 ∘C
for rutile inclusions and 688–700 ∘C for matrix rutile. For
sample 03-59, the thermometer gives T ranges of 684–784 ∘C at peak
P of 2.4 GPa and 639–734 ∘C at 1.3 GPa. Uncertainty of the
thermometry has been estimated to be ∼30 ∘C
(Tomkins et al., 2007).
DiscussionPrograde to peak P stages
Retrieving the exact prograde P–T path for the studied samples is difficult
because much of the prograde information has been lost during melting at
high T; however, P–T pseudosections modeled with melt-reintegrated compositions
could yield critical information on the prograde path (e.g., Indares et
al., 2008). Although one-step or multi-step approaches have been proposed to
reintegrate melt compositions, they yield similar overall topologies of the
modeled phase diagrams (Bartoli, 2017). By a one-step
melt-reintegration approach, the modeled pseudosection (Fig. 9) shows that a
few restricted fields with paragonite occur on the prograde path. Abundant
crystallized melt pockets with albite found in garnet, interpreted as being formed
through melting of paragonite inclusions (see Sect. 7.2.1 for detailed
discussion), require the former presence of paragonite during garnet
growth. Thus, the prograde P–T path is interpreted to traverse
paragonite-present P–T fields at 1.3–2.1 GPa, 600–740 ∘C (field A
in Fig. 10).
Previous studies suggested a broad range for peak P of the NEGEP: 1.8–2.3
GPa for garnet websterite from the central block (Brueckner et al.,
1998) and 1.6–2.0 GPa for kyanite eclogite from Weinschenk Island
(∼20 km to the south of Sanddal) in the central block
(Elvevold and Gilotti, 2000). This study used forward isochemical phase
equilibrium modeling along with mineral isopleths to generate reliable P–T
conditions based on mineral composition (Massonne, 2013; Powell and
Holland, 2008). The melt-reintegrated pseudosection, along with pristine
phengite with average high Si (3.24–3.27 p.f.u.) and omphacite with
average high XNa (0.43–0.44), yielded peak P of 2.4±0.1 GPa at
830±30∘C (Fig. 9). This estimated P–T range is located
across the solidus; therefore, it is likely that a small fraction of melt
(<7 vol %) was already present at peak P. The P–T estimates are well
within the range of error for garnet-clinopyroxene-kyanite-phengite-quartz
thermobarometry (2.57±0.32 GPa, 818±65∘C).
The prograde to peak P path is also supported by temperature estimated from
Zr-in-rutile thermometry. The rutile in both garnet and in the matrix is
interpreted to have formed from prograde to potentially maximum T at higher
P, because pseudosections using both the measured and melt-reintegrated bulk
composition do not show the presence of rutile at lower P conditions (Figs. 7, 9). The rutile in garnet yielded temperatures of 674–726 ∘C
at 1.3 GPa and 723–777 ∘C at 2.4 GPa, which are within the
prograde conditions estimated on the basis of the paragonite-present fields.
Matrix rutile yields a T range at 737–749 ∘C in sample 03-57 and
684–784 ∘C in sample 03-59 at 2.4 GPa (i.e., at peak P). This T
range is slightly lower than the estimates from pseudosections, similar to
previous research using this thermometry (e.g., Tual et al., 2018; Liu et
al., 2015); this discrepancy can be attributed to diffusion modification of
Zr concentration, kinetic effects in chemical disequilibrium, inaccuracy in
the thermometry calibration or the application of pseudosection modeling or
a combination of these processes (e.g., Tual et al., 2018; Liu et al., 2015).
Exhumation and partial melting stagesEvidence of partial melting
Concordant and discordant leucocratic veins within the eclogite pod and
foliated eclogite in the margin are mesoscopic evidence that partial melting
occurred in these rocks. The thickness of the leucosomes varies from millimeter to
centimeter scales; the leucosomes merge from thinner into thicker veins (Fig. 2),
which indicate that they originated internally. Potential melt channels
(e.g., Fig. 3b) suggest that modes could have reached the threshold
(7 %; Rosenberg and Handy, 2005) for melt loss. Zoisite eclogite 03-59
from the margin of the pod has a strong L-S fabric defined by
quartzofeldspathic layers and elongate zoisite grains (Figs. 2, 3) that
possibly represents melt migration channels, with the original composition
altered by melt loss to (or melt gain from) adjacent parts.
At the microscopic scale, areas marked by interstitial plagioclase,
peritectic minerals and graphic intergrowth are interpreted as former melt
pockets. Interstitial plagioclase in the crystallized melt pockets shows low
dihedral angles (<60∘) at plagioclase solid–solid
boundaries and is interpreted as original melt–solid boundaries, where
plagioclase formed by crystallization of melt at surrounding solids such as
garnet, zoisite or clinopyroxene (Holness and Sawyer, 2008).
Such cuspate boundaries are present in all studied samples (Fig. 3),
regardless of the degree of melting. Small euhedral garnet (e.g., in 03-59)
adjacent to plagioclase with typical cuspate contacts and subhedral garnet
with euhedral crystal faces in 03-57 are interpreted to have formed and/or grown as a
peritectic phase during partial melting. Graphic amphibole and plagioclase
intergrowths crystallized from a hydrous melt because undercooling can lead
to epitaxial growth of minerals along earlier formed grains (e.g.,
Fenn, 1986; London and Morgan, 2012). The quartzofeldspathic layers in
sample 03-59 contain abundant euhedral garnet crystals and cuspate
boundaries with plagioclase that formed during partial melting and
subsequent cooling.
Polymineralic inclusions in garnet in HP and UHP eclogites have been
interpreted to form via various mechanisms, for example, partial melting
after entrapment of inclusion (Perchuk et al., 2005, 2008; Cao et al.,
2019), trapped and crystallized melt (Ferrero et al., 2015; Chen et al.,
2014; Ferrando et al., 2005; Stöckhert et al., 2001, 2009; Liu et al., 2013), and reaction between infiltrating
fluids and precursor minerals (Liu et al., 2018). The polymineralic
inclusions do not contain complex inclusion assemblages from fluid leaching
of the host mineral and thus are not products of fluid interaction but melt
crystallization. These inclusions are present throughout Grt I; if
polymineralic inclusions represent trapped and crystallized melt, the
inclusion process would require significant garnet growth (>20 vol %) at melt-present conditions, which is not the case for the studied
eclogites (5 vol %–7 vol % increase, according to the modeling results; see Fig. S1c). Therefore, the polymineralic inclusions are
interpreted as crystallized melt generated from in situ melting of
preexisting minerals in garnet. The polymineralic inclusions typically
contain albite, K-feldspar, kyanite, phengite and quartz (Fig. 4), which
requires the preexisting minerals to contain significant Na and K. A
single phase of a nominally anhydrous mineral such as plagioclase, omphacite
or K-feldspar is unlikely to melt at the peak conditions; the hydrous
minerals paragonite and phengite are strong candidates to form Na- and
K-rich melt. Melting of the inclusions led to volume expansion, which
cracked the host mineral (Fig. 4). Melt migrated along intragranular
fractures into the matrix, leaving restitic minerals in the cracks (Fig. 3d). Droplets of melt migrated into the matrix adjacent to large garnet and
crystallized, forming sharp offshoots into host minerals (Fig. 3d). New
garnet was generated by melting and modified by high-T diffusion, forming the
observed patchy zoning (Fig. 5; Perchuk et al., 2008).
Melting reactions
The exact melting reactions cannot be determined from the polymineralic
inclusions in garnet (Fig. 4) due to the lack of original reactant phases.
Documented mineral phases and the P–T history predicted by the pseudosections
are needed. Crystallized melt pockets in the matrix commonly contain
reactants and products, and thus can be used to interpret melting reactions
using pseudosections.
Melting via paragonite and phengite breakdown occurred in garnet. Both
minerals are predicted on the prograde path by the melt-reintegrated
pseudosection (Fig. 9) and can be captured along with other minerals during
garnet growth and preserved in the robust host mineral at higher P. Melting
of paragonite and phengite with additional minerals occurred in garnet after
reaching their breakdown temperatures and generated melt, kyanite and other
minerals, for instance, by the following reactions: white mica + quartz →
K-feldspar + kyanite + melt (Lang and Gilotti, 2007; Huang and
Wyllie, 1974) and phengite + clinopyroxene + quartz → garnet +
kyanite + melt (Hermann and Green, 2001).
In contrast, phengite and zoisite breakdown melting occurred in the matrix.
A few crystallized melt pockets (e.g., Fig. 3d) with a granitic composition
and sharp offshoots are interpreted as crystallized melt derived from
phengite breakdown. The mineral assemblage Grt II + Ky + Kfs + Ab can
form via the following reaction: phengite + clinopyroxene + quartz → garnet
+ kyanite + melt (Hermann and Green, 2001), where the melt
crystallizes to albite and K-feldspar in pockets. The abundant crystallized
pockets of melt consisting of amphibole and plagioclase that are present
between anhedral zoisite and diopside (Fig. 3b, c) suggest that the
eclogites were partially melted through the following reaction: zoisite + omphacite
+ quartz → melt + Na-poor clinopyroxene + garnet. Zoisite and
omphacite were the remaining reactants during the partial melting, whereas
diopside, euhedral garnet and melt were products. The melt later
crystallized as oligoclase (XAn=0.29) and pargasitic amphibole.
This zoisite melting reaction is similar to those suggested by Skjerlie
and Patiño Douce (2002).
Partial melting conditions
The zoisite eclogites have undergone multi-stage partial melting, as
suggested by textures and pseudosections. One stage of partial melting may
have occurred only in garnet after paragonite entrapment at P–T conditions
outside of paragonite-present fields (Fig. 10); however, the exact P–T could
not be determined. The inclusions could have been shielded by the strong
host mineral, avoiding pressure-induced breakdown, but then melted in
response to increasing temperature. Phengite in both garnet and the matrix
may have melted to form peritectic garnet, which then captured residual
phengite grains. The initial melting may have occurred at peak P=2.4 GPa
via phengite breakdown, according to the melt-reintegrated pseudosection (B in Fig. 10).
Another important stage of partial melting involves zoisite breakdown.
Zoisite is predicted to be absent at peak P using the melt-reintegrated
pseudosection (Fig. 9) but is present in the studied rocks, indicating its
formation on the exhumation path (Fig. 7). Zoisite typically forms a corona
around kyanite, suggesting consumption of the aluminosilicate and Ca from
other minerals or the melt. Such reactions would be reflected as decreasing
modal kyanite and increasing modal zoisite, which is predicted at 1.9–2.1 GPa, 800–900 ∘C. With continued exhumation, zoisite melts along
with additional phases (e.g., omphacite), generating calcic melt at P<1.9 GPa (Figs. S1 and 10c).
The availability of fluid in the shear zone may have been critical in
determining different degrees of partial melting in different parts of the
outcrop. The zoisite eclogite 03-57, collected from the inner part of the
eclogite pod, experienced a lower degree of partial melting, as evidenced by
the smaller number of crystallized melt pockets. With limited availability
of free fluid, the core of the eclogite pod partially melted through the
in situ and localized breakdown of hydrous phases. In contrast, sample 03-59 from
the margin of the pod shows a strong fabric, which might be due to combined
fluid-absent and fluid-present melting. The outer eclogite pod might have
suffered fluid-absent hydrous mineral-breakdown melting, evidenced by
peritectic garnet, forming leucocratic layers. Fluid may have also
infiltrated the eclogite pod, triggering partial melting and forming high
modal zoisite, essentially generating a higher degree of partial melting
than the center of the pod.
Cooling stage
Cooling of the HP terrane led to crystallization of melt pockets both in
garnet and matrix. At peak T (∼900∘C), the garnet
rim equilibrated with the bulk rock. At least >7 vol % of melt
was also formed (Fig. 7), perhaps leading to extraction along melt channels.
That the melt pockets crystallized at pressures below the jadeite + quartz
= albite reaction is indicated by the presence of plagioclase (Fig. 3).
The cooling rate from peak T was probably low, and the presence of graphic
intergrowths points to undercooling, typical for epitaxial growth of
minerals (London and Morgan, 2012). Empirical hornblende-plagioclase
thermometry yielded a temperature of ∼750∘C, which is close to the solidus on the modeled pseudosection (Fig. 7).
Vermicular symplectite of diopside, plagioclase and amphibole replaced
omphacite during this stage.
Implications for eclogite melting
Hydrous minerals are critical to partial melting of metabasite at elevated
pressure due to the absence of free water in orogenic lower crust. This
process is commonly termed dehydration melting because the breakdown of
OH-bearing minerals releases free water into the system, triggering partial
melting of the host rock. Phengite is regarded as an important K-bearing
phase that leads to partial melting of metabasite at HP and UHP, as
demonstrated by experimental studies (Liu et al., 2009; Skjerlie and
Patiño Douce, 2002) and in natural metabasites (Wang et al., 2014;
Deng et al., 2019). Paragonite, common in medium-temperature HP eclogites
(Massonne and Sobiech, 2007), is absent in the studied eclogite
but is predicted on the melt-reintegrated pseudosection (Fig. 9) and
documented in eclogite from nearby Weinschenk Island (Fig. 1; Elvevold and
Gilotti, 2000). This Na-rich mica accounts for the albite-bearing
polymineralic inclusions and Na-rich melt in the studied samples and has
been argued to be a major contributor to partial melting of UHP eclogite
from the Sulu belt, eastern China (Chen et al., 2014). Similarly,
epidote-group minerals can break down and induce partial melting of calcic
eclogite. The contribution of this melting depends on bulk composition.
Melting by zoisite breakdown in the Sulu UHP eclogites was
insignificant (Chen et al., 2014), but an epidote-group mineral is
critical to melting UHP kyanite eclogite (Cao et al., 2019)
and the studied zoisite eclogites. The remarkably preserved crystallized
melt pockets in the studied samples indicate that zoisite was a significant
contributor to anatexis. Zoisite with domains of growth and resorption in HP
pegmatite from the Variscan Münchberg Massif (Germany) was interpreted
to have formed through heating on the exhumation path (Liebscher et al., 2007).
Thermodynamic models of minerals and tonalitic melt (Green et al., 2016) provided a critical
framework and extended the modeling capabilities to melting of metabasites (Hernández-Uribe et al., 2020; Palin et al., 2016a, b); however,
problems still exist. Phase relations of phengite, zoisite and melt in the
modeled pseudosection (Fig. 10) show discrepancies with the
theoretically calculated (Vielzeuf and Schmidt, 2001) and
experimentally determined P–T boundaries (Lopez and Castro, 2001; Liu et
al., 2009; Skjerlie and Patiño Douce, 2002) of these phases in
metabasite. The phase relations vary with bulk rock and mineral
composition. For example, Lopez and Castro (2001) used an amphibolite
as a starting material, while Skjerlie and Patiño Douce (2002) did
experiments on zoisite eclogite and Liu et al. (2009) did the same on
phengite eclogite. The bulk composition of the studied eclogites, being
similar to starting materials in Skjerlie and Patiño Douce (2002),
yielded a pseudosection with large discrepancies in the upper thermal limits
of zoisite + melt (Fig. 10) compared to the experimental results. The upper
thermal stability of zoisite in experiments and theoretical calculations
differs by >100∘C (Skjerlie and Patiño Douce,
2002; Vielzeuf and Schmidt, 2001) for specific mafic bulk compositions,
indicating the need for further examination. Wei and Duan (2018)
compared phase relations of melt and other phases for a mid-ocean ridge basalt (MORB)-based
pseudosection with various experiments and showed that the stability fields
are consistent for amphibole and epidote at suprasolidus; however,
considerable discrepancies are present at subsolidus, which shows shrunken
epidote and enlarged amphibole fields. Forshaw et al. (2019) attributed
discrepancies between observed and predicted values in amphibole and
clinopyroxene chemistry from high-temperature metabasite to incorrect
element partitioning and substitution vectors by the models. The current
thermodynamic dataset and solution models for metabasite needs further
development to correctly model the relevant phase relations.
Concluding summary
Zoisite eclogites from the Sanddal area of the NEGEP were partially melted
at near-peak P and on the exhumation path. The mineral assemblage at peak P was garnet, omphacite, kyanite, zoisite, phengite, quartz and
rutile. Isochemical phase equilibrium modeling of the melt-reintegrated rock
composition, along with isopleths of Si-in-phengite and
XNa-in-clinopyroxene, yields peak-P conditions of 2.4±0.1 GPa at
830±30∘C. Partial melting is indicated by microscopic
and mesoscopic melt-related textures. Graphic amphibole-plagioclase
intergrowths and small garnet neoblasts adjacent to anhedral zoisite and
clinopyroxene with plagioclase cusps point to the principal reaction being zoisite
+ clinopyroxene = melt + garnet. Polymineralic inclusions of
K-feldspar, albite and other phases with sharp offshoots into garnet hosts
crystallized from melt derived from breakdown of phengite and paragonite
previously enclosed in garnet. A small percentage of melt loss occurred,
leading to formation of leucocratic veins within the eclogite pod and
foliated eclogite at the rim. A pseudosection of a melt domain, along with
garnet and plagioclase isopleths, yields a peak T of 900±50∘C at 1.9±0.2 GPa following peak P. Phase equilibrium
modeling of the melt-reintegrated composition further corroborated the
presence of paragonite on the prograde path and yielded peak conditions
similar to the ones using the bulk rock. The eclogite reached an equilibrium
condition of 1.3 GPa and 750 ∘C, determined by thermobarometry
using amphibole and plagioclase formed on the retrograde path.
Data availability
Additional mineral chemistry will be provided upon request.
The supplement related to this article is available online at: https://doi.org/10.5194/ejm-32-405-2020-supplement.
Author contributions
JAG collected the samples and secured major funding for the research. JAG,
HJM and WTC designed the research project. WTC carried out detailed
petrography, mineral chemistry, thermobarometry and phase equilibrium
modeling. WTC prepared the manuscript with significant contributions from
the other co-authors.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
The authors thank Thomas Theye for his help with the electron microprobe
analysis and Simona Ferrando, Tom Foster, Chun-Jing Wei and Giselle Rebay for valuable discussions and comments on an earlier version of the
paper. We also wish to thank Aphrodite Indares and an anonymous
reviewer for constructive comments, and Chiara Groppo and Elisabetta Rampone for editorial handling and comments that have significantly improved this paper. Publication of the study was supported by the GPMR research grant (GPMR201818) to Wentao Cao. Wentao Cao appreciates the support of the T. Anne Cleary Dissertation Research Fellowship, Post-comprehensive Summer Research Fellowship, and Ballard and Seashore Dissertation Fellowship from the Graduate College, University of Iowa.
Financial support
This research has been supported by the National Science Foundation, Division of Earth Sciences (grant no. 1049433).
Review statement
This paper was edited by Chiara Groppo and reviewed by Aphrodite Indares, Chiara Groppo, and one anonymous referee.
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