Deciphering protoliths of the (U)HP rocks in the Makbal metamorphic complex, Kyrgyzstan: geochemistry and SHRIMP zircon geochronology

The (ultra)high-pressure (U)HP Makbal metamorphic complex (MMC) belongs to a chain of early Paleozoic (U)HP terranes to the south of the Paleo-Kazakhstan continent, a major component of the Central Asian Orogenic Belt. We present geochemical characteristics, reconstructions of geodynamic settings and constraints on the emplacement and depositional ages of metamorphosed intrusive and sedimentary rocks of the MMC. The MMC comprises variably metamorphosed juxtaposed packages of mica schists, hosting boudins of metamorphosed mafic rocks, metaquartzites and metacarbonates in a tectonic melange. The rocks, metamorphosed under (U)HP conditions, are situated in the central part of the terrane. The UHP garnet talc chloritoid schists are believed to have formed from immature pelitic sediments with maximum depositional ages of 700-650 Ma. The (U)HP metaquartzites comprise mature sediments derived from a Paleoproterozoic continental source. They were formed in a passive-margin environment and have a maximum depositional age of ca. 1600 Ma. The chemical composition of the MMC metabasic rocks is compatible with subalkaline tholeiitic basalts with E-MORB compositions; however, some samples have N-MORB and OIB affinities. The ages of zircon xenocrysts from the metabasic rocks show that the mafic intrusions are probably younger than ca. 700 Ma. The metabasic rocks represent dikes and sills emplaced into immature pelitic sediments in a rift-related or continental-slope environment at the final stages of the Rodinia break up (700-600 Ma) when shallow mantle melting produced relatively primitive tholeiitic compositions. The whole sequence, including the quartzites, was later juxtaposed with other passive-margin sediments and metamorphosed under (U)HP conditions in a Caledonian subduction zone between 500 and 475 Ma. The ages of detrital zircon grains from the MMC rocks indicate derivation from a Paleoproterozoic continental source and match the age pattern characteristic for the Northern Tien Shan, with major peaks at 1.95-1.75 Ga and smaller peaks between 2.7 and 2.5 Ga and at ca. 1.0 Ga. The ages between 1.65 and 1.4 Ga are uncharacteristic for the Kyrgyz Northern Tien Shan, instead may be correlated with recently documented voluminous early Mesoproterozoic arc magmatism in the Central Tien Shan Arc Terrane in China. The resetting of the U-Th-Pb system and the growth of new zircon in the MMC rocks were probably defined by the permeability to aqueous fluids. An age of 456 4 Ma, obtained for the rims of zircon xenocrysts from a small amphibolite boudin in the southern part of the MMC, corresponds with the age of an adjacent Ordovician granite and indicates that Paleozoic intrusions could have provided sufficient heat to cause resetting and/or growth of new zircon in the MMC rocks.