Oblique continental collision and the formation of syn-collisional A-type granites: insights from the Early Jurassic Baoji granite suite in the Qinling orogenic belt, central China

A-type granites are conventionally interpreted to be formed in late post-collisional or anorogenic settings. However, we firstly identified an Early Jurassic syn-collisional A-type granite suite in the Qinling orogenic belt: the Baoji granite suite. A comprehensive investigation into its mineralogical, petrological, geochronological, and geochemical characteristics can shed light on the genesis of syn-collisional A-type granites and geodynamic evolution of collisional orogeny. The Baoji granite suite contains two types of rocks: (1) A-type granites including biotite syenite, coarse-grained, porphyritic, and fine-grained syenogranites, and alkali-feldspar granite, and (2) magnesian monzodiorite; both types are coeval, with zircon U–Pb ages of ca. 190 Ma. The A-type granites yield A₁-type trace element signatures and large variations in trace element contents, with Ba contents of 16.9–762 ppm, Sr of 11.0–211 ppm, Rb of 193–645 ppm, and Eu of 0.08–1.24 ppm. Their radiogenic isotope compositions (⁸⁷Sr/⁸⁶Sr_i = 0.70631–0.70637; <i>ε</i>Nd = −5.18 to −3.84; εHf = −5.01 to +2.76) are similar, and overlap those of Neoproterozoic OIB-like basic volcanics from the South Qinling belt. These characteristics show that the A-type granites originated from the anatexis of OIB-like lower crust and underwent fractional crystallization in a silicic magma reservoir. The monzodiorite has low SiO₂ contents (49.82–53.29 wt.%) and enriched radiogenic isotopic compositions (⁸⁷Sr/⁸⁶Sr_i = 0.70554–0.70638; <i>ε</i>Nd = −11.0 to −5.28) contains abundant hornblende and biotite (~40%), and exhibits evidence of magma mingling. It also contains two groups of zircon: pristine and unzoned zircon that yields negative Hf isotope compositions (<i>ε</i>Hf = −9.42 to −3.14), and xenocrystic zircon that yields similar Hf isotope compositions (<i>ε</i>Hf = −3.54 to +1.85) to the A-type granites. These characteristics demonstrate that the monzodiorite was derived from the differentiation of basaltic partial melts of an enriched mantle source, and mingled with the A-type granitic magma during its ascent and emplacement into a silicic magma reservoir. Along with regional records of the Early Jurassic magmatism, deformation, metamorphism, and sedimentation, the genetic models for the Baoji A-type granites and monzodiorite suggest that the Middle Triassic to Early Jurassic oblique continental collision (ca. 230–190 Ma) produced coupled EW-trending strike-slip and NE-trending extensional faults at crustal levels and Rayleigh–Taylor (gravitational) convective instabilities at mantle depths in the Qinling orogenic belt, which controlled the production of the syn-collisional A-type granite suite. Our work also indicates that the A-type granites can be formed in syn-collisional setting in response to oblique continental collision and the resultant slow foundering of a high-density lithospheric root; therefore, we suggest that the syn-collisional A-type granites are evidence for oblique continental collision.传统观点认为，A型花岗岩（A-type granites）形成于碰撞后晚期或非造山环境。然而，我们首次在秦岭造山带识别出早侏罗世同碰撞A型花岗岩套——宝鸡花岗岩套。对其矿物学、岩石学、年代学及地球化学特征开展的综合研究，有助于阐明同碰撞A型花岗岩的成因与碰撞造山带的地球动力学演化过程。宝鸡花岗岩套包含两类岩石：（1）A型花岗岩，包括黑云母正长岩、粗粒斑状正长花岗岩、细粒正长花岗岩以及碱长花岗岩；（2）镁铁质二长闪长岩；二者均为同期形成，锆石U-Pb年龄（zircon U–Pb ages）约为190 Ma。A型花岗岩具有A₁型微量元素地球化学特征，且微量元素含量变化范围较大，其中Ba含量为16.9~762 ppm，Sr为11.0~211 ppm，Rb为193~645 ppm，Eu为0.08~1.24 ppm。其放射成因同位素组成（⁸⁷Sr/⁸⁶Sr_i = 0.70631~0.70637；εNd = -5.18~-3.84；εHf = -5.01~+2.76）较为均一，且与南秦岭带新元古代类洋岛玄武岩（OIB）基性火山岩的同位素组成重叠。上述特征表明，该A型花岗岩起源于类OIB下地壳的深熔作用，并在长英质岩浆房中经历了分离结晶作用。该镁铁质二长闪长岩的二氧化硅（SiO₂）含量较低（49.82~53.29 wt.%），且富集放射成因同位素（⁸⁷Sr/⁸⁶Sr_i = 0.70554~0.70638；εNd = -11.0~-5.28），含有大量角闪石和黑云母（约占40%），并表现出岩浆混合作用的证据。其锆石可分为两类：一类为未分带的原生锆石，εHf值为-9.42~-3.14；另一类为捕掳晶锆石，其εHf值（-3.54~+1.85）与A型花岗岩的Hf同位素组成相似。上述特征表明，该二长闪长岩起源于富集地幔源区的玄武质部分熔融体的分异作用，并在上升并侵位进入长英质岩浆房的过程中，与A型花岗岩浆发生了混合。结合区域早侏罗世岩浆作用、变形作用、变质作用及沉积作用的记录，宝鸡A型花岗岩与二长闪长岩的成因模型表明，中三叠世至早侏罗世的斜向大陆碰撞（约230~190 Ma）在秦岭造山带的地壳层面形成了共轭的东西向走滑断裂与北东向伸展断裂，同时在幔源深度产生了瑞利-泰勒（重力）对流不稳定，这一构造动力学过程控制了同碰撞A型花岗岩套的形成。本研究还表明，A型花岗岩可形成于响应斜向大陆碰撞以及由此引发的高密度岩石圈根缓慢沉陷的同碰撞环境中；据此我们提出，同碰撞A型花岗岩可作为斜向大陆碰撞的判别证据。