Multistage Metamorphic Evolution of Retrograded Eclogites from the Songshugou Complex, Qinling Orogenic Belt, China

Abstract

Abstract The Qinling Orogenic Belt is one of the major collisional orogens in eastern Asia and marks the boundary between the North China Craton and South China Craton. The Songshugou complex is the largest basic to ultrabasic body to be found in the North Qinling Belt, and was emplaced as a lens-shaped body at the southern margin of the Qinling Group. A detailed petrological investigation of garnet amphibolite, augen amphibolite and well-foliated amphibolite together with garnet zoning patterns of major and trace elements, inclusions in garnet, and thermodynamic modelling indicate a multistage metamorphic history. Garnets clearly show characteristics of discontinuous growth, as they display optically light-colored snowball-textured cores surrounded by a darker mantle with few inclusions as well as chemically a sudden increase in grossular and decrease in almandine components. A partly resorbed rim is not recognized optically but mineral inclusions and a discontinuous chemical composition of garnet are proof of this third garnet growth stage. Rare earth element distribution patterns of garnet also show clear evidence for discontinuous growth and allow us to identify the reactions responsible for garnet growth. Garnet core compositions as well as amphibole inclusions allow us to constrain a P–T window where this rock equilibrated in a first stage. Calculated pseudosections and the application of the garnet–amphibole thermometer indicate an upper amphibolite- to lower granulite-facies metamorphic episode at 630–740 °C and 0·7–0·9 GPa. The presence of relict omphacite as well as a discontinuously grown garnet mantle with rutile inclusions clearly places the peak metamorphic stage in the eclogite facies. Garnet (XGrs, XAlm, XPrp) and omphacite isopleths (XMg, XNa) constrain this event at 1·7–2·1 GPa and 570–650 °C. Consistent temperatures of 500–650 °C were also determined by clinopyroxene–garnet geothermobarometry for this event. Growth of an outermost rim as well as different stages of garnet breakdown to plagioclase + amphibole coronae and the nearly complete replacement of former omphacite by a variety of symplectites point to an intricate retrograde P–T path. In more strongly retrograded samples plagioclase + amphibole ± quartz pseudomorphs entirely replace former garnet grains. Certain coronae around garnets and symplectites also contain prehnite and pumpellyite, which formed during a late retrograde stage or during a different event at very low P–T conditions (250–350 °C). Based on the detailed petrological study, we favour a multistage metamorphic history of the Songshugou metabasic rocks. The age of the eclogite-facies metamorphic event must be related to the deep subduction of the Songshugou complex during the early Paleozoic, although the age of garnet core growth remains enigmatic. The development of garnet cores indicates an upper amphibolite-facies regional metamorphic overprint succeeded by an eclogite-facies event around 500 Ma and subsequent retrogression seen in replacement of garnet and formation of symplectite. The latest imprint evidenced by prehnite and pumpellyite may be the result of fluid infiltration during the fading orogenic phase or represents a low-temperature overprint by a later process, probably related to the uplift of the North Qinling terrane at around 420 Ma.