Petrogenesis and tectonic implications of Early Cretaceous high-K calc-alkaline volcanic rocks in the Laiyang Basin of the Sulu Belt, eastern China

摘要

Early Cretaceous high-K calc-alkaline volcanism occurring in the Laiyang Basin north of the Sulu high-pressure to ultrahigh-pressure (HP-UHP) Metamorphic Belt, eastern China, comprises a wide spectrum of rock types, ranging from trachybasalts to trachydacites. The basaltic-andesitic rocks erupted at 107-105 Ma, spanning an SiO2 range of 50.1-59.6% and an MgO range of 2.6-7.2%, and are characterized by large ion lithophile element (LILE; e.g. Ba and K) and light rare earth element (LREE) enrichment, high field strength element (HFSE) depletion and highly radiogenic Sr but non-radiogenic Nd isotopic compositions (Sr-87/Sr-86(i) = 0.70750-0.70931; epsilon(Nd)(t) = -17.9 - -15.6). The geochemical similarities between these rocks and the earlier Sulu Belt lamprophyres suggest that both types of mafic rocks were derived from similar mantle sources with LILE and LREE enrichment. Thus, the Wulian-Qingdao-Yantai Fault that separates the two terranes at the surface should not be considered as a lithospheric boundary between the North China and Yangtze blocks. The felsic lavas erupted at 93-91 Ma, spanning an SiO2 range of 61.6-67.0% and an MgO range of 1.1-2.6%, and show a trace element geochemistry similar to the basaltic rocks, but with higher radiogenic Sr and even lower Nd isotopic compositions (Sr-87/Sr-86(i) = 0.70957-0.71109; epsilon(Nd)(t) = -19.1 - -17.5), similar to I-type granitoids in the Sulu Belt. A crustal origin was proposed to explain their compositions (which are comparable to those of experimental slab melts), the > 10 Ma eruption interval and the compositional gaps in some elements (e.g. P, Ti and Sr) between them and the older basaltic-andesitic rocks. These melts were derived from predominant metaigneous protoliths containing mafic accumulative counterparts of the basaltic-andesitic and/or lamprophyric magmas. The extensive extrusion of Early Cretaceous high-K calc-alkaline rocks in the Laiyang Basin favored an extensional regime in response to the progressive attenuation of the thickened lithosphere and orogenic collapse, as reflected in the development of the basin from a foreland basin (before the end of the Jurassic period) to a fault basin (since the Early Cretaceous period).