Abstract
Abstract. 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.
Highlights
Partial melting, along with subsequent melt extraction and magma ascent, leads to geochemical differentiation in large orogens (e.g., Clemens, 2006)
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
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)
Summary
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 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, 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
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