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Metamorphic textures and P–T evolution of high‐ P granulites from the Lelukuau terrane, NE Grenville Province
Author(s) -
Indares A. D.
Publication year - 2003
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1046/j.1525-1314.2003.00414.x
Subject(s) - kyanite , granulite , geology , geochemistry , plagioclase , anorthosite , igneous rock , metamorphic rock , metamorphism , terrane , petrology , quartz , geomorphology , tectonics , facies , paleontology , structural basin
High‐pressure (HP) granulites and eclogitized metagabbro are exposed along an orogen‐parallel high‐ P belt that was developed at c. 1050–1020 Ma in the NE Grenville Province. Among these rocks, mafic granulites derived from a Labradorian anorthosite suite of the Lelukuau terrane contain garnet, Al‐Na diopside, and, depending on bulk composition, plagioclase and kyanite. Moreover, the distribution of phases is influenced by the original igneous texture. For instance, in high X MgO leucocratic varieties, garnet porphyroblasts nucleated together with kyanite in An‐rich cores of plagioclase domains whereas in low X MgO rocks garnet occurs together with clinopyroxene within formerly igneous ferromagnesian domains and kyanite is missing. In contrast, garnet pseudomorphs after igneous plagioclase in melanocratic varieties display evidence of earlier corona development. Metamorphic textures are consistent with a two stage evolution: (a) development of garnet and Al‐Na‐diopside (Cpx 1 ) under high‐ P metamorphic conditions, concomitant with elimination of plagioclase in the mesocratic to melanocratic varieties; and (b) partial loss of Al‐Na from Cpx 1 resulting in production of new andesitic plagioclase, and growth of new clinopyroxene (Cpx 2 ) after garnet and quartz in leucocratic to mesocratic rocks consistent with decompression. Widespread equilibrium textures between garnet‐Pl 2 ‐Cpx 2 and/or reset Cpx 1 are consistent with development at the thermal peak. Estimated P–T conditions for the presumed thermal peak fall in the range 1500–1800 MPa and 800–900 °C and are comparable to those recorded by eclogitized gabbros from other parts of the high ‐P belt of the NE Grenville province. Low jadeite content of clinopyroxene from the HP granulites is attributed to the low bulk Na 2 O/(Na 2 O + CaO) of these rocks relative to common basaltic compositions. Scarcity of apparent retrograde textural overprint in both the HP granulites and the eclogites suggests fast subsequent cooling, consistent with extrusion of the high ‐P belt towards the foreland shortly after the metamorphic peak.