Petrological and geochemical characteristics of the Wanganzhen complex and discussion on its genesis

The Wanganzhen complex is one of representative example of Taihang-Da Hinggan tectonomagmatic belt in eastern China, which has the largest scale and the most rock types. It consists of three-stage rocks. According to the major element, trace element, REE and Nd, Sr, Pb isotopic characteristics of the Wanganzhen complex reported in this paper, the main stage rocks (granodiorite, adamellite and few quartz diorite) have the similar petrological and geochemical characteristics to those of adakite with SiO2≥58%, Al2 O3≥15%, low Y content (4.20∼12, 68μg/g), high Sr content (518∼861μg/g) and Sr/Y ratios (60. 19∼178, 10), LREE enrichment with (La/Yb)N = 34. 06∼76.91, HREE depletion with Yb=0.44∼1.00, no obviously Eu anomaly (Eu/Eu°=0.83∼1.03 , and relative depletion in the high field strength elements (such as Nb, Hf, Ti), ISe=0.7060, negative εNd (t) value(-16.29∼-14.27) and positive εSr (t) value (+23.7∼+25.0), but obviously different from those of subduction-related adakites. These adakite-like rocks were most likely derived from the partial melting of basaltic lower crust in the thickened crust resulted from hot mantle material upwelling as several mushroom cloud and the thermally altered ground uplifting to the bottom of continental crust during late Jurassic period. But the early and the late stage ones belong to non-adakites. The early basic- to intermediate- instrusive rocks (hornblendite, gabbroite and diorite) are high Mg and Fe, LREE enrichment, Eu positive anomaly, enriched in compatible elements and low incompatible elements, ISr=0.7056, εNd(t)=-16. 72∼ -10.92, which are the products of partial melting of subcontinental lithospheric enrich mantle and probably represent the underplating basaltic magma below the thickened continental crust during early Mesozoic era. The late alkaline intermediate- to acid-intrusive rocks (alkali-granite, quartz syenite and quartz monzonite) are characteristic of high K and Na, alkaline series and high Σ REE, negative Eu anomaly, low Sr content, high Y and Yb, negative Sr anomaly, ISr = 0. 7062, εNd (t)=-16.25, suggesting that the formation of adakite-like rocks of the main stage result in the lithospheric delamination and cause asthenosphere to upwell to the bottom of thinned continental crust, so the rocks of the late stage are formed by the partial melting of the materials in the crust-mantle transformation belt even in the middle crust. In this paper the Rb-Sr isochronal age of granodiorite (145.7±5.3Ma) is reported. Based on the Rb-Sr isochronal age of granite and granidiorite, the main body of the Wanganzhen complex was probably formed in late Jurassic period between 137.8Ma and 145.7Ma.