Metamorphic evolution of the Naga Hills eclogite and blueschist, Northeast India: implications for early subduction of the Indian plate under the Burma microplate

Abstract

Tectonic slices and lenses of eclogite within mafic and ultramafic rocks of the Early Cretaceous–Eocene Naga Hills ophiolite were studied to constrain the physical conditions of eastward subduction of the Indian plate under the Burma microplate and convergence rate prior to the India–Eurasia collision. Some of the lenses are composed of eclogite, garnet‐blueschist, glaucophanite and greenschist from core to margin, representing a retrograde hydrothermal alteration sequence. Barroisite, garnet, omphacite and epidote with minor chlorite, phengite, rutile and quartz constitute the peak metamorphic assemblage. In eclogite and garnet‐blueschist, garnet shows an increase in Mg and Fe and decrease in Mn from core to rim. In chlorite in eclogite, Mg increases from core to rim. Inclusions of epidote, glaucophane, omphacite and quartz in garnet represent the pre‐peak assemblage. Glaucophane also occurs profusely at the rims of barroisite. The matrix glaucophane and epidote represent the post‐peak assemblage. The Fe3+ content of garnet‐hosted omphacite is higher than that of matrix omphacite, and Fe3+ increases from core to rim in matrix glaucophane. Albite occurs in late stage veins. P–T pseudosection analysis indicates that the Naga Hills eclogites followed a clockwise P–T path with prograde metamorphism beginning at ∼1.3 GPa/525 °C and peaking at 1.7–2.0 GPa/580–610 °C, and subsequent retrogression to ∼1.1 GPa/540 °C. A comparison of these P–T conditions with numerical thermal models of plate subduction indicates that the Naga Hills eclogites probably formed near the top of the subducting crust with convergence rates of ∼ 55–100 km Myr−1, consistent with high pre‐collision convergence rates between India and Eurasia.

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“…Other areas are known to have eclogites, associated with continental subduction, such as the Tso Morari Complex in Ladakh (Donaldson et al, 2013) and Kagan in Pakistan (Tonarini et al, 1993), but these do not yield large volumes of glaucophane, and in the case of Tso Morari, were likely not areas of rapid erosion since the late Miocene. Other suture zone blueschists are eroded into the Brahmaputra and are not relevant here (Chatterjee and Ghose, 2010). This provenance demonstrates a direct link between the main stream of the Indus River and the fan at Sites U1456 and U1457.…”

On ultrahigh temperature crustal metamorphism: Phase equilibria, trace element thermometry, bulk composition, heat sources, timescales and tectonic settings

“…Sapphirine is typically indicative of high to extreme crustal temperatures (e.g. Hensen, 1971Hensen, , 1972bHensen, , 1986Hensen, , 1987Green, 1971, 1973;Chatterjee and Schreyer, 1972;Newton 1972;Newton et al, 1974;Ellis et al, 1980;Grew, 1980;Harley, 1985Harley, , 1986Sandiford, 1985a;Sandiford et al, 1987;Powell and Sandiford, 1988;Bertrand et al, 1991;Motoyoshi et al, 1993;Audibert et al, 1995;Das et al, 2001Das et al, , 2003Hollis and Harley, 2003;Podlesskii et al, 2008;Podlesskii, 2010), with sapphirine stability in silica-undersaturated rock compositions expanded to lower temperatures than for sapphirine þ quartz bearing assemblages (e.g. Schreyer and Seifert, 1969a;Seifert, 1974;Ackermand et al, 1975;Hensen, 1987;Kelsey et al, 2005).…”

“…Mapping (or point counting) local domains in a thin section and combining the abundance of minerals in that mapped area with analysed mineral compositions to derive a domain composition is a method by which the PeT information of individual mineral reaction microstructures can be investigated (e.g. Friedman, 1960; White et al, 2003;Corvino et al, 2007;Chatterjee and Ghose, 2010;Cutts et al, 2011aCutts et al, , 2013Forbes et al, 2011;Droop and Brodie, 2012;Anderson et al, 2013;Walsh et al, 2014). Alternatively, quantitative mapping of local domains in a thin section can be undertaken, negating the need to combine maps with separately measured mineral compositions (e.g.…”

“…Ultrahigh temperature (UHT) metamorphism is the most thermally extreme type of regional crustal metamorphism, defined by Harley (1998b) as non-igneous crustal temperatures above 900 C. UHT metamorphism of continental crust is now widely accepted as a relatively commondrather than anomalousdcharacteristic of deep crustal processes, greatly assisted by the large (and growing) number of localities worldwide that contain mineral assemblages whose stability has been constrained experimentally (Schreyer and Seifert, 1969a,b;Hensen and Green, 1970;Hensen, 1971Hensen, , 1972aHensen, ,b, 1977Hensen and Essene, 1971;Hensen and Green, 1971, 1972, 1973Chatterjee and Schreyer, 1972;Newton 1972;Doroshev and Malinovskiy, 1974;Newton et al, 1974;Seifert, 1974;Ackermand et al, 1975;Kiseleva, 1976;Arima and Onuma, 1977;Annersten and Seifert, 1981;Bertrand et al, 1991;Motoyoshi et al, 1993;Audibert et al, 1995;Carrington, 1995;Carrington andHarley, 1995a,b, 1996;Das et al, 2001Das et al, , 2003Hollis and Harley, 2003;Brigida et al, 2007;Fockenberg, 2008;Podlesskii et al, 2008;Podlesskii, 2010) and reinforced by calculated phase equilibria (Kelsey et al, 2004;Kelsey et al, 2005;Podlesskii et al, 2008;Podlesskii, 2010;Taylor-Jones and Powell, 2010;Holland and Powell, 2011;…”

Tectonics and Evolution of the Trans-Himalayan Mountains and Nagaland Ophiolite Belt

“…2B) is surrounded by successive layers of glaucophane schist, glaucophanite and greenschist (Chatterjee and Ghose, 2010). These authors deduced a prograde clockwise P-T path (13 kb, 525°C), peak (17-20 kb, 580-610°C) for the eclogites (Fig.…”