华北陆块南缘华山和合峪花岗岩岩体锆石U-Pb年龄、Hf同位素组成与成岩动力学背景

Zircon U-Pb age and Hf isotope composition of the Huashan and Heyu granite plutons at the southern margin of North China Craton: Implications for geodynamic setting.

华山和合峪花岗岩体分别位于华北陆块南缘的西北部和东南部,华山岩体顶部混染相二长花岗岩的LA-ICP-MS锆石U-Pb年龄为133.8±1.1 Ma,合峪复式杂岩体第三次侵入的似斑状黑云母二长花岗岩的LA-ICP-Ms锆石U-Pb年龄为134.5±1.5Ma,二者年龄相近。华山岩体与合峪岩体的地球化学特征相似,其SiO_2>69.0%,Al_2O_3>13.0%,K_2O+Na_2O>7.0%,Na_2O>3.2%,ACNK<1.1,具有高钾钙碱性花岗岩类的特征。两岩体的轻重稀土元素分馏显著,Sr含量较高(Sr多数>400×10^(-6),Y、Yb含量低(Y<18×10^(-6),Yb<2×10^(-6)),Eu负异常较弱(δEu>0.67),HILE富集,HFSE亏损,表明岩浆房存在斜长石+角闪石+石榴石+金红石的分离结晶,或者这些矿物在源区部分熔融时作为残留相。华山岩体岩浆锆石ε_(Hf)(t)集中在-18～-20之间,Hf二阶段模式年龄t_(DM2)集中于2.1～1.8Ga;合峪岩体岩浆锆石ε_(Hf)(t)集中在-16～-17之间,Hf二阶段模式年龄t_(DM2)集中于2.0～1.7Ga。以上表明,两岩体均为增厚下地壳(2.1～1.7Ga左右)部分熔融形成的陆壳改造型花岗岩类。综合区域地质演化,我们认为,侏罗纪及其以前的碰撞挤压或逆冲推覆使地壳增厚,侏罗纪一白垩纪之交的挤压向伸展转换过程的减压增温条件导致加厚下地壳(2.1～1.7Ga左右)部分熔融,岩浆上升侵位造成了华山岩体及合峪岩体等碰撞改造型花岗岩类的发育。

The Huashan and Heyu multistage granite complexes occur in the northwestern and southeastern portions of the southern margin of the North China Craton, respectively. The contamination phase monzonitic granite at top of the Hushan complex and the third stage porphyric biotite monzonitic granite of the Heyu complex yield LA-ICP-MS zircon U-Pb weighted average ages of 133.8 +/- 1.1 Ma and 134.5 +/- 1.5 Ma, respectively. These two plutons share similar geochemical features. Both of them belong to high K cal-alkaline granites, with SiO2 > 69.0%, Al2O3 > 13.0%, K2O + Na2O > 7.0%, Na2O > 3.2%, and ACNK < 1.1; obvious fractionation between LREE and HREE; high Sr contents (mostly Sr > 400 x 10(-6)) and low Y and Yb contents (Y < 18 x 10(-6), Yb < 2 x 10(-6)); slightly negative Eu anomaly (delta Eu > 0.67); and enrichment in LILE, and depletion in HFSE (Nb, Ta and Ti). This indicates the fractionation of plagioclase + hornblende + garnet + rutile assemblage from the primary magmas or residence during partial melting of magma source. Zircons of two plutons show similar Hf isotopic compositions. The epsilon(Hf)(t) values of the Huashan pluton range from -20 to -18, with t(DM2) ages between 2.1 and 1.8Ga; while the epsilon(Hf)(t) values of the Heyu pluton from -17 to -16, with most t(DM2) ages of 2.0 similar to 1.7Ga. This suggests that both plutons were formed by partial melting of the thickened lower crust (2.1 similar to 1.7Ga). In combination with the regional tectonic evolution, we argue that the crust in the study area was thickened by collisional compression or overthrusting during Jurassic and ealier lime, and then the conditions of decompression and geothermal increasing during transition from Jurassic compression to Cretaceous extension result in partial melting of the thickend lower crust, and thereby the collisional transformation-type granites such as the Huashan and Heyu complexes formed at the southern margin of the North China Craton.