Fluid-enhanced melting during prograde metamorphism

Journal of the Geological Society, Mar 2001 by Prince, C, Harris, N, Vance, D

Abstract: Anatexis is a commonly recognized feature of high-grade metamorphism, but segregated melts are generally ascribed to anatexis during peak metamorphic conditions and little is known about melting along the prograde path. A suite of small-volume, deformed, two-mica leucogranites has been recognized within the High Himalayan Crystalline Series of the Garhwal Himalaya. These granites are consistently more siliceous than minimum-melt granite compositions and are characterized by low Rb/Sr ratios, high Ba, low abundances of HFS elements and positive Eu anomalies. Such trace-element characteristics contrast strongly with the geochemistry of the well-studied Early Miocene leucogranites of the High Himalaya, derived from fluid-absent melting. Sm Nd garnet dating of one deformed granite indicates a crystallization age of 39 /- 3 Ma, c. 15 Ma before the emplacement of the more voluminous High Himalayan leucogranites. Whilst some entrainment of restitic phases cannot be excluded, trace element signatures suggest a low temperature (

Fluid-enhanced melting may be a common feature of prograde upper amphibolite-facies metamorphism of orogenic belts, predating peak metamorphism by at least 15 Ma. These melts will only crystallize within this period if they segregate from their protoliths. Subsequent dating of long-lived melts would indicate erroneously young ages for the prograde melting events. However, melts formed in this way may be recognized by their distinctive trace-element chemistry. The persistence of early formed melts within an orogen provides insights into the prograde heating path, and may be critical in controlling the rheology of the middle crust, and hence its deformational history.

Keywords: Himalaya, anatexis, fluids, granites, tectonics.

Crustal melting is a widely documented consequence of prograde heating within high-grade orogenic zones. Although such melts can form either under fluid-present conditions or during mica and hornblende dehydration reactions (Brown & Fyfe 1970; Le Breton & Thompson 1988) the high solubility of H20 in granite magmas suggests that the volumes of H20saturated melts that can form will be limited by the flux of aqueous fluids into the melt zone (Clemens & Vielzeuf 1987). Consequently, fluid-present melting is an unlikely mechanism for generating large volumes of granites that have segregated from their source regions, particularly at deeper crustal levels.

In the Himalayan orogen, crustally derived leucogranites have been widely recognized intruding the High Himalayan Crystalline Series that constitutes the high-grade core of the orogen (Le Fort et aL 1987). These late or post-kinematic bodies were emplaced between 24 and 17 Ma (Harrison et al 1998) and are largely undeformed, although movement on the South Tibetan Detachment System has resulted in local deformation of some leucogranites. Their origin has been variously ascribed to fluid infiltration from Main Central Thrust (Le Fort et al. 1987), shear heating along that thrust (England et al. 1992) or to decompression melting during movement on the detachment zone (Harris & Massey 1994). A study of their trace element systematics suggest a pelitic source that partially melted under fluid-absent conditions (Inger & Harris 1993).

In this study we describe some strongly deformed granites within the High Himalayan Crystalline Series that provide geochemical and chronometric constraints on fluid-enhanced melting in the metasedimentary rocks of the Garhwal Himalaya. These melts significantly predate the generation of the more substantial High Himalayan Leucogranites.

Analytical work in the UK was supported by NERC Research Grant GR3/10919 and a NERC fellowship to D.V. Field work in India was supported by Talat Ahmad at the Wadia Institute, a Royal Society Research Grant (N.H.) and a NERC Research Studentship (C.P.). We particularly wish to thank V.C. Thakur, Director of the Wadia Institute, for providing support throughout our field programme and Felix Oberli for use of the mass spectometer at ETH Zurich. We gratefully acknowledge the help of John Watson for assistance with XRF analyses.

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