西大别桥店花岗斑岩脉的形成时代、岩石成因及其大地构造意义

白垩纪碰撞后花岗岩是研究大别造山带碰撞后伸展垮塌的重要载体,西大别南缘的桥店花岗斑岩脉为约束大别山碰撞后构造机制转化提供了新的约束信息。SIMS和LA-ICP-MS锆石定年结果表明,桥店花岗斑岩脉侵位年龄约为129 Ma,并具有富集的锆石Hf同位素组成(ε_Hf(t)=-24.5-14.7)和古元古代二阶段模式年龄(2.41.9 Ga)。矿物组成上,花岗斑岩脉以富含粗粒的长石斑晶为主要特征。它们具有变化的SiO₂含量(63.07%73.22%)和A/CNK值(0.871.73),同时具有高的K₂O(4.51%5.47%)、低的MgO (0.42%1.82%)含量,属于高钾钙碱性花岗岩类。岩石的轻稀土元素相对重稀土元素富集,具有Eu的负异常(Eu/Eu^*=0.770.92);同时富集Rb、Ba和Pb,而亏损Nb、Ta和Ti。相较于典型埃达克质岩石,桥店花岗斑岩的Sr含量(78×10^-6724 ×10 ^-6)变化较大,Y(11.8×10^-614.8×10^-6)和Yb(1.09×10^-61.37×10^-6)含量相对较高,对应的Sr/Y(6.755.5)和(La/Yb)_N(29.634.2)比值较低。以上地球化学特征,结合古元古代的残留锆石和二阶段铪模式年龄,共同反映出桥店花岗斑岩是区内古元古代下地壳物质在中-低压力条件下部分熔融的产物。与区域上碰撞后岩浆岩的对比研究表明,桥店花岗斑岩的侵位指示了西大别地壳在约129 Ma已经开始减薄,西大别白垩纪加厚下地壳的拆沉及构造机制转换的时间可能在约130 Ma前后。

Geochronology,petrogenesis and tectonic implications of the Qiaodian granite porphyry from the western Dabie Orogenic Belt,Central China

The ca. 145-120 Ma granitoids were regarded as important products of the thickened, delaminated partial melting Cretaceous crust beneath the Dabie Orogenic Belt. Our newly identified Qiaodian granite porphyry in the western Dabie Orogenic Belt can provide new constraints on the progression of lithospheric thinning and the transformation of tectonic regime to an extensional setting. SIMS and LA-ICP-MS zircon U-Pb dating suggests the Qiaodian granite porphyry was emplaced at 129 Ma. It has a negative zircon ε_Hf(t) value (between -24.5 and -14.7) with a two-stage model age of 2.4-1.9 Ga, and is characterized by coarse-grained feldspar phenocrysts. It has variable SiO₂ contents (63.07%-73.22%) and A/CNK values (0.87-1.73), with high K₂O(4.51%-5.47%) and low MgO(0.42%-1.82%) contents, suggesting high-K calc-alkaline granite affinity. It exhibits LREE-enriched rare earth elemental distribution patterns ((La/Yb)_N=29.6-34.2) with negative Eu anomalies ((Eu/Eu ^*)=0.77-0.92). The primitive mantle-normalized trace element patterns show enrichments of Rb, Ba and Pb and depletions of Nb, Ta and Ti. Compared with typical adakites, the Qiaodian granite porphyry has variable Sr((78-724)×10^-6) and relatively high Y((11.8-14.8)×10^-6) and Yb((1.09-1.37)×10^-6) contents, yielding a low Sr/Y ratio (6.7-55.5). These geochemical features, together with Paleoproterozoic residual zircon and two-stage Hf model ages, indicate the Qiaodian granite porphyry mainly derived from partial melting of Paleoproterozoic mafic materials at medium to low pressures. On the basis of our new results and previously published data, we consider the tectonic collapse and transformation of the western Dabie Orogenic Belt probably occurred around 130 Ma.