The early Yanshanian geology of Southeastern (SE) China is characterized by widespread igneous rocks consisting predominantly of granites and rhyolites, and subordinate mafic intrusive and volcanic rocks. However, the onset time, petrogenesis, and tectonic implications of the early Jurassic magmatism in SE China remain highly controversial.

The scientists of Inst. of Geochemistry, Chinese Academy of Sciences (IGCAS), report the zircon U–Pb ages, geochemistry, and Nd–Sr isotopic data for the Fe–Ti–V oxide-bearing Xianlan mafic–ultramafic intrusion and the Wengong granitic pluton in northeast Guangdong. SHRIMP and Cameca SIMS U–Pb zircon ages indicate that the Xialan gabbros and the Wengong granites were emplaced at 194 ± 1 Ma and 192 ± 1 Ma, respectively. Although the field observations show that the granitic pluton intruded the mafic–ultramafic intrusion, the dating results suggest that they were formed almost contemporaneously.

The ore-barren gabbros of the Xialan intrusion are commonly high in Fe2O3, CaO and Al2O3 contents, with variable SiO2, TiO2, and MgO contents. In contrast, the Fe–Ti–V-bearing gabbros have higher Fe2O3, TiO2 (3.5–5.4%) and V contents. These gabbros with high εNd(T) values (+ 1.7 to + 6.2) and low initial ISr values (0.704 to 0.706), are characterized by LREE-enriched and “convex upwards” incompatible trace-element patterns with slightly negative Nb–Ta anomalies. The parental magma for the gabbros exhibits affinity with a high-Ti subalkaline basaltic magma generated by melting of a depleted OIB-like mantle source. The scientists suggest that the gabbros originated by fractional crystallization of the parental magma plus varying degrees of crustal contamination.

The Wengong granites have high SiO2 and (Na2O + K2O) contents, are LREE-enriched and show relatively flat-HREE patterns and significantly negative Eu, Nb, Sr, P and Ti anomalies in the primitive mantle-normalized spidergrams. These granitic rocks display geochemical characteristics of A2-type granites including high FeOT/MgO ratios (7.6–14.7), elevated high-field-strength element (HFSE) contents, and high Ga/Al ratios (2.75–3.49). The Wengong granites exhibit variably initial εNd(T) values ranging from − 2.8 to + 1.2, whereas the initial ISr values show large uncertainties due to relatively high Rb/Sr ratios. The parental magma for these rocks exhibits affinity with a mildly alkaline magma, which was probably generated by partial melting of the regional Paleoproterozoic amphibolites and thereafter mixed with various amounts of the early Jurassic basaltic magmas. The granites were then formed by extensive crystal fractionation of the mildly alkaline magma.

The Xialan mafic–ultramafic intrusion and the Wengong granitic pluton were formed in an anorogenic extensional environment. It is therefore suggested that ca. 194 Ma is an important onset timing of widespread anorogenic magmatism in SE China. The scientists interpret the early Jurassic mafic–ultramafic intrusion and A-type granites to represent an anorogenic magmatism in response to an onset of asthenospheric mantle upwelling due to the break-up of a subducted flat-slab beneath the SE China continent.

 

Publication Name	Lithos, Volume 119, Issues 3-4, October 2010, Pages 313-329
Author(s)	Zhu WG,  Zhong H, Li XH , He DF, Song XY, Ren T, Chen ZQ , Sun HS , Liao JQ
Corresponding Author	ZHU Weiguang,  zhuweiguang@vip.gyig.ac.cn,  Chinese Acad Sci, Inst Geochem, State Key Lab Ore Deposit Geochem, 46 Guanshui Rd, Guiyang 550002, Peoples R China

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