矿床地球化学国家重点实验室知识库

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. We report here 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 ε_Nd(T) values (+ 1.7 to + 6.2) and low initial I_Sr 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. We 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 FeO^T/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 ε_Nd(T) values ranging from − 2.8 to + 1.2, whereas the initial I_Sr 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. We 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.