Anatectic record and contrasting P-T paths of aluminous gneisses from the central Grenville Province

作者：Lasalle S^*; Indares A

来源：Journal of Metamorphic Geology, 2014, 32(6): 627-646.

DOI：10.1111/jmg.12083

• anatectic Al-gneisses
• granulites
• Grenville Province
• P-T pseudosections
• THERMOCALC

Anatectic aluminous gneisses, some derived from sedimentary rocks of broadly pelitic composition and others from hydrothermally altered felsic volcanic rocks, are exposed in the mid-P and high-P segments of the hinterland in the central Grenville Province. These gneisses consist dominantly of garnet, biotite, K-feldspar, plagioclase and quartz, with sillimanite or kyanite, and display microstructural evidence of anatexis by fluid-absent reactions consuming muscovite and/or biotite. Melt-related microstructures, such as inter-granular films and/or interstitial quartz or feldspar enclosing relict phases, are most abundant in the metasedimentary samples. Despite anatexis at granulite facies conditions, the hydrothermally altered rocks preserve earlier features attributed to the circulation of hydrothermal fluids, such as sillimanite seams, dismembered quartz veins and garnet-rich aluminous nodules in a K-feldspar-dominated matrix. Microstructural and mineral chemical data, integrated with P-T pseudosections calculated with THERMOCALC for the metasedimentary rocks, permit qualitative constraints on the P-T paths. Data from a high-P kyanite-bearing sample are consistent with a steep prograde P-T path up to similar to 14.5 kbar and 860-900 degrees C, followed by decompression with minor cooling to the solidus at similar to 11 kbar and 870 degrees C. This pressure-dominated P-T path is similar to those inferred in other parts of the high-P segment in the central Grenville Province. In contrast, the P-T path predicted from a mid-P sillimanite-bearing paragneiss has a strong temperature gradient with P-T of similar to 9.5 kbar and 850 degrees C at the thermal peak, and a retrograde portion down to similar to 8 kbar and 820 degrees C. In a broad sense, these two contrasting P-T patterns are consistent with predictions of thermo-mechanical modelling of large hot orogens in which P-T paths with strong pressure gradients exhume deeper rocks in the orogenic flanks, whereas P-T paths with strong temperature gradients in the orogenic core reflect protracted lateral transport of ductile crust beneath a plateau.

• 出版日期2014-8