Massive granitoid generation in Central Asia: Nd isotope evidence and implication for continental growth in the Phanerozoic

Abstract

The Central Asian Orogenic Belt (CAOB), also known as the Altaid Tectonic Collage, is characterized by vast distribution of Paleozoic and Mesozoic granitic intrusions as well as basaltic to rhyolitic volcanics. The granitoids have a wide range of compositions and roughly show a temporal evolution from calc-alkaline, alkaline to peralkaline series. The emplacement times for most granitic plutons fall between 500 to 120 Ma, but only a small proportion of plutons have been precisely dated. In this paper we document the available Nd isotopic data to advocate that massive juvenile continental crust was generated during the Phanerozoic in Central Asia. Most Phanerozoic granitoids of Central Asia are characterized by low initial Sr isotopic ratios, positive εNd(T) values and young Sm-Nd model ages (TDM) of 300 to 1200 Ma. This is in strong contrast with the coeval granitoids emplaced in some "classic" orogenic belts, such as, the European Caledonides and Hercynides, Cathaysia of SE China and South Korea. The isotope data indicate their "juvenile" character and suggest their derivation from source rocks or magmas separated shortly from the upper mantle. Granitoids with negative εNd(T) values also exist, but they occur in the environs of Precambrian blocks and their isotope compositions reflect contamination of the older crust in the magma generation processes. The evolution of the CAOB is undoubtedly related to accretion of young arc complexes and old terranes (microcontinents). However, the emplacement of large volumes of post-tectonic granites requires another mechanism - probably through a series of processes including underplating of massive basaltic magma, intercalation of basaltic magma with lower crustal granulites, partial melting of the mixed lithologic assemblages leading to generation of granitic liquids, followed by extensive fractional crystallization. The proportions of the juvenile or mantle component for most granitoids of Central Asia are estimated to vary from 70 to 100%. The recent "discovery" of juvenile crust in many Phanerozoic orogenic belts elsewhere, together with the CAOB, may considerably change our views of the continental growth.