Constraints on the Proterozoic evolution of the Aravalli-Delhi Orogenic belt (NW India) from monazite geochronology and mineral trace element geochemistry

摘要

The timing and extent of polymetamorphism in the Mangalwar and Sandmata Complexes from the Aravalli-Delhi Orogenic Belt of Rajasthan (NW India) remains contentious, with Archaean, Paleoproterozoic and Neoproterozoic events having previously been postulated. Monazite SHRIMP U-Pb and electron microprobe (EPMA) chemical ages obtained from metasediments in the amphibolite-facies Mangalwar Complex show that it was metamorphosed at ca. 0.97-93 Ga, with evidence in one sample for an earlier event at ca. 1.82 Ga. Monazite and garnet REE patterns from metapelitic and metapsammitic rocks are characterised by small negative Eu anomalies, suggesting that they experienced amphibolite-facies conditions during both events. In contrast, granulite-facies metamorphism in the Sandmata Complex occurred at ca. 1.72 Ga, although the monazite U-Pb system was partially disturbed during a localised high-strain overprint that occurred at ca. 1 Ga. A comparison of the REE patterns of porphyroclastic and neoblastic garnets in the Sandmata Complex rocks shows that the second event occurred at amphibolite-facies conditions, consistent with Zr thermometry of rutile in the shear fabric. Garnet REE patterns show that relict granulite-facies garnet porphyroclasts are present even in Sandmata samples so sheared and rehydrated that they are now amphibolite-facies schists. REE patterns of isotopically disturbed Sandmata Complex monazite suggest that the age variations generally reflect partial Pb loss due to solid-state deformation in the later shear zones. New (ca. 1 Ga) monazite growth under amphibolite-facies conditions occurred only in the most intensely recrystallised and rehydrated rock. In metapelitic rocks the magnitude of Eu anomalies developed in garnet and accessory phases such as monazite is particularly sensitive as to whether K-feldspar grew at the same time. In the case of the Sandmata granulites this distinction makes it possible to determine abundance of relict granulite-facies, and newly formed amphibolite-facies garnet, in extensively sheared, rehydrated and recrystallised granulites, and the extent of new monazite growth during the shearing event