Growth, preservation of Paleoproterozoic-shear-zone-hosted monazite, north of the Western Dharwar Craton (India), and implications for Gondwanaland assembly

Abstract

We examine the conditions and processes of growth and preservation of multiaged monazite in micaceous matrix and in garnet porphyroblasts in staurolite–kyanite mica schists hosted in a hitherto-undiscovered shear zone that limits the northern extent of the Western Dharwar Craton (WDC), India. Garnet in the footwall schists grew during mid-crustal (600 ± 40 °C, 7.3 ± 1.2 kbar) loading and cooling as a consequence of the northward transport of the WDC lithologies. U–Th–Pb (total) ages in monazites in the matrix and in post-tectonic garnets yield well-defined peaks at 2.5, 2.2 and 1.9 Ga. In garnet, 2.5 and 2.2 Ga monazite grains, and 2.2 Ga monazites with 2.5 Ga cores are commonly occluded, but monazites with 1.9 Ga mantles around older cores are rare. By contrast, in the matrix, 1.9 Ga monazite grains and monazite with 1.9 Ga mantles around older cores are prominent, but the peak age frequencies of the two older populations are significantly lower than for monazites hosted as inclusions in garnet. Both in the matrix and garnet, the low-Th, high-Y domains in monazites yield the two older peak ages, while the 1.9 Ga ages correspond to the high-Th, low-Y domains. The preponderance of older ages in monazite hosted as inclusions in garnet relative to matrix monazites is because garnets formed between 2.2 and 1.9 Ga shielded the older monazites from dissolution–precipitation at 1.9 Ga. A few 1.9 Ga monazites hosted as inclusions in the garnet rims suggest renewed garnet growth at post-1.9 Ga. Multiple Pb–Pb age populations (2.5, 2.25, 2.1 and 1.8 Ga) in detrital zircon in the Sahanataha Group north of the Paleoarchean Antongil-Masora block (NE Madagascar) are identical to the multiple monazites ages north of the WDC, inferred to share a similar history and to be contiguous with the Antongil-Masora block in pre-Jurassic reconstructions of the Gondwanaland. We suggest the newly discovered Paleoproterozoic tectonic zone continued westward into Madagascar north of the Antongil-Masora block and constituted the hitherto-unexplained basement for the multiaged detrital zircons in the Sahanataha quartzites (337).