The Late Cretaceous I and A type granite association of southeast China: Implications for the origin and evolution of post collisional extensional magmatism
Abstract We present new geochronological, mineralogical, and geochemical data for granitic plutons that crop out within the Zhoushan archipelago, northeastern coastal Zhejiang Province, in order to constrain their origin, and the genetic relationship between the I and A type granites. These granites can be divided into two groups: (1) the northern I type Putuoshan (PTS) and Dadong'ao (DDA) plutons; and (2) the southern A type Daqingshan (DQS) Taohuadao (THD) and Xiazhidao (XZD) plutons. Zircon LA ICP MS U Pb dating yielded ages of 98 96 Ma for the northern I type plutons and 89 86 Ma for the southern A type plutons. All of these granites are highly siliceous, K rich, and have similar total alkali and total rare earth element (REE) abundances. However, there are also geochemical differences between the I type and the A type granites. The northern I type alkali feldspar granites are high K calc alkaline, metaluminous to mildly peraluminous, contain low concentrations of the high field strength elements (HFSE; e.g. Nb, Ta, Zr, and Hf) and have low Ga/Al ratios (2.04 2.44) In contrast, the southern A type granites are peralkaline and F rich, and have lower CaO and Al2O3 concentrations, and higher Fe2O3T and HFSE concentrations and Ga/Al ratios (3.25 3.86) Meanwhile, they have slightly higher heavy REE (HREE) concentrations, and are more depleted in Ba, Sr, P, Ti, and Eu than the northern I type granites. Both the I and A type granites have homogeneous whole rock Nd and highly variable zircon Hf isotopic compositions. Of note, the southern peralkaline A type granites appear to have more radiogenic Nd and Hf isotope compositions than the northern I type granites. The present data, together with the results of a previous study on mafic enclaves within the PTS pluton, suggest that the northern I type alkali feldspar granites were generated by mixing of mantle derived material with crustal derived magmas that formed by dehydration melting of mica bearing basaltic rocks, leaving a granulite residue in the lower crust. Both the I and A type granites were emplaced during post collisional extensional tectonism associated with rollback of the steeply subducting paleo Pacific Plate. An increase of the dip angle of the subducted palaeo Pacific plate between the Early and Late Cretaceous resulted in enhanced extension, further thinning the lithosphere and causing more intensive underplating of mantle derived magmas. This generated high temperatures and caused the partial melting of the residual granulite material, producing relatively anhydrous F bearing felsic melts. The mixing of these unusual crustal melts with more voluminous mantle derived mafic magmas generated the parental magmas that eventually formed the peralkaline A type granites.