GEMOC - Department of EPS - Faculty of Science

MANTLE EVOLUTION IN THE EAST CENTRAL ASIA OROGENIC BELT: HEAT SOURCES FOR GRANITE GENERATION

W.L. Griffin^1,2 S.Y. O'Reilly¹ , D.A. Ionov¹ , Y.H. Poudjom Djomani¹ , V.G. Malkovets^3,1

1. GEMOC Macquarie
2. CSIRO Exploration and Mining
3. Inst. of Mineralogy and Petrography, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090, Russia

Mantle-derived xenoliths in alkali basalts provide spot samples of the subcontinental lithospheric mantle (SCLM) at several locations across the East Central Asia Orogenic Belt (ECAOB). We have examined large suites from the Vitim, Dariganga, NW Mongolia and Minusa areas, all of which are in areas of juvenile Phanerozoic crust, or older crust strongly reworked during the Phanerozoic assembly of the orogenic belt. No xenolith samples from beneath unambiguous "microcontinents", where Proterozoic SCLM could be preserved, have yet been studied. Despite subtle differences, these xenolith suites show common features relevant to SCLM structure and the generation of the granites of the region. All show a SCLM column <100 km thick, with a high advective geotherm typical of intraplate alkali-basaltic volcanism. In most localities the SCLM consists of spinel peridotite from the crust-mantle boundary to depths of 45-60 km, with garnet±spinel peridotites at greater depth. Regional variations in the average degree of depletion are reflected in variations in the depth of the spinel-garnet peridotite transition, which is shallowest beneath the Minusa area and deepest beneath the Dariganga Plateau.

Most of the SCLM sections sampled here show chemical stratification. Garnet peridotites generally show low degrees of depletion (¾5% partial melting), as reflected in flat REE patterns in clinopyroxenes, high abundances of cpx+gnt and CaO and Al2O3 contents typically 2.5-4 wt. %. These rocks are interpreted as underplated asthenospheric material, and published Nd model ages of 1.7-2.0 Ga may reflect the mean age of primary depletion in the convecting mantle, rather than a specific depletion event. Many of the spinel peridotites also are quite fertile, but some show higher degrees of depletion, followed by metasomatic enrichment in LREE and incompatible elements such as U, Th, Sr and Zr. These samples may represent older lithosphere. The accretion history of the ECAOB, with the successive closing of ocean basins, is not reflected in the xenolith suites; none is depleted enough to represent typical oceanic or island-arc mantle.

Density modelling shows that cool oceanic or sub-arc mantle is not gravitationally stable if <60 km thick; it can be subducted, or delaminated under compressive stress. Detachment of this depleted lithosphere will allow upwelling of hotter asthenospheric material, providing a large, regionally distributed heat source. If the geotherm during this process approximates the Cenozoic xenolith-derived geotherm (probably a minimum estimate), temperatures will reach 900-1000 °C at the crust-mantle boundary and 700-800 °C in the middle crust, causing massive melting, granitoid production and basification in the lower crust. This mechanism could explain the large volumes of late/post-tectonic granitoids intruded across the ECAOB. The present SCLM structure beneath the areas sampled is interpreted as consisting of (1) shallow thin remnants of older continental or oceanic lithospheric mantle mixed with younger asthenospheric material, and (2) deeper underplated "asthenospheric" material, probably modified by minor further melting during upwelling. This model predicts that some minor basaltic/gabbroic magmatism should accompany granite intrusion.