Data for: Zircon U-Pb, Geochemical and Isotopic Constraints on the Age and Origin of A- and I-type Granites and Gabbro-Diorites from NW Iran: Implication for Continental Crust Growth

The continental crust of NW Iran is intruded by Late Cretaceous I-type granites and gabbro-diorites as well as Paleocene A-type granites. SIMS and LA-ICPMS U-Pb analyses of zircons yield ages of 100-92 Ma (Late Cretaceous) for I-type granites and gabbro-diorites and 61-63 Ma (Paleocene) for A-type granites. Late Cretaceous gabbro-diorites (including mafic microgranular enclaves; MMEs) from NW Iran show variably evolved signatures. They show depletion in Nb and Ta on N-MORB-normalized trace element spider-diagrams and have high Th/Yb ratios, suggesting their precursor magmas were generated in a subduction-related environment. Gabbro-diorites have variable zircon εHf(t) values of +1.2 to +8, δ18O of 6.4 to 7.4 ‰ and bulk rock εNd(t) of -1.4 to ~ +4.9. The geochemical and isotopic data attest to melting of subcontinental lithospheric mantle (SCLM) to generate near-primitive gabbros with radiogenic Nd isotope (εNd(t)= ~ +4.9) and high Nb/Ta and Zr/Hf ratios, similar to mantle melts (Nb/Ta~17 and Zr/Hf~38). These mafic melts underwent further fractionation and mixing with crustal melts to generate Late Cretaceous evolved gabbro-diorites. Geochemical data for I-type granites indicate both Nb-Ta negative and positive anomalies along with enrichment in light REEs. These rocks are peraluminous and have variable bulk-rock εNd(t) (-1.4 to +1.3), zircon εHf(t) (+2.8 to +10.4) and δ18O (4.7-7.3 ‰) values, but radiogenic bulk rock Pb isotopes. The geochemical and isotopic signature of these granites suggest interaction of mantle-derived mafic magmas (similar to near-primitive Oshnavieh gabbros) with middle-upper crust through assimilation-fractional crystallization (AFC) to produce Late Cretaceous I-type granites. Paleocene A-type granites have distinct geochemical features compared to I-type granitoids, including enrichment in Nb-Ta, high bulk rock εNd(t) (+3.3 to +3.9) and zircon εHf(t) (+5.1- +9.9) values. Alkaline granites are ferroan; they have low MgO, CaO, Sr, Ba and Eu concentrations and high total Fe2O3, K2O, Na2O, Al2O3, Ga, Zr, Nb-Ta, Th and rare earth element (REE) abundances and Ga/Al ratios. These rocks might be related to fractionation of a melt resulting from a sub-continental lithospheric mantle, but with interaction with asthenosphere-derived melts. We suggest that subduction initiation and the resultant slab roll-back caused extreme extension in the overlying Iranian plate, induced convection in the mantle wedge and led to the compression melting of SCLM. Rising mantle-derived magmas assimilated middle-upper crust. Fractionating mantle-derived magmas and contamination with crustal components produced evolved gabbro-diorites and I-type granites. In contrast, asthenosphere upwelling during Paleocene provided heat and melt for melting of- and interacting with SCLM to generate A-type granite precursor melts.

伊朗西北部大陆地壳发育晚白垩世I型花岗岩、闪长辉长岩以及古新世A型花岗岩侵入体。对锆石开展的二次离子质谱（SIMS）与激光剥蚀电感耦合等离子体质谱（LA-ICPMS）U-Pb定年分析结果显示，I型花岗岩与闪长辉长岩的形成年龄为100~92 Ma（晚白垩世），A型花岗岩的形成年龄为61~63 Ma（古新世）。伊朗西北部的晚白垩世闪长辉长岩（包括镁铁质微粒包体（MMEs））呈现出不同程度的演化特征：在正常型大洋中脊（N-MORB）标准化的微量元素蛛网图中，这类岩石表现出Nb、Ta亏损，且Th/Yb比值较高，表明其母岩浆形成于俯冲相关环境。闪长辉长岩的锆石εHf(t)值介于+1.2至+8之间，δ18O值为6.4‰~7.4‰，全岩εNd(t)值为-1.4至~+4.9。地球化学与同位素数据证实，大陆下岩石圈地幔（SCLM）的熔融形成了近乎原始的辉长岩，其具有放射成因Nd同位素（εNd(t)≈+4.9）以及较高的Nb/Ta和Zr/Hf比值，与地幔熔体的特征（Nb/Ta≈17，Zr/Hf≈38）高度相似。这些镁铁质熔体经历了进一步的分异作用，并与地壳熔体混合，最终形成了演化程度较高的晚白垩世闪长辉长岩。I型花岗岩的地球化学数据显示其兼具Nb-Ta负异常与正异常，且轻稀土元素（REE）富集。这类岩石属于过铝质，全岩εNd(t)值介于-1.4至+1.3之间，锆石εHf(t)值为+2.8~+10.4，δ18O值为4.7‰~7.3‰，同时具有放射成因的全岩Pb同位素组成。上述地球化学与同位素特征表明，地幔来源的镁铁质岩浆（与近乎原始的Oshnavieh辉长岩相似）通过同化-分离结晶（AFC）作用与中上部地壳发生相互作用，从而形成了晚白垩世I型花岗岩。与I型花岗岩类相比，古新世A型花岗岩具有独特的地球化学特征：包括Nb-Ta元素富集，较高的全岩εNd(t)值（+3.3~+3.9）与锆石εHf(t)值（+5.1~+9.9）。碱性花岗岩属于铁碱质系列，其MgO、CaO、Sr、Ba及Eu浓度较低，而全铁以Fe2O3计、K2O、Na2O、Al2O3、Ga、Zr、Nb-Ta、Th及稀土元素（REE）丰度与Ga/Al比值较高。这类岩石可能源于大陆下岩石圈地幔熔融形成的熔体发生分异作用，但过程中受到了软流圈来源熔体的混染。我们认为，俯冲作用的启动及其引发的板片回撤导致上覆伊朗板块发生强烈伸展作用，诱发地幔楔对流，并引发大陆下岩石圈地幔的减压熔融。上升的地幔源岩浆同化了中上部地壳，分异的地幔源岩浆与地壳组分的混染作用形成了演化程度较高的闪长辉长岩与I型花岗岩。与之相反，古新世时期的软流圈上涌为大陆下岩石圈地幔的熔融及其相互作用提供了热量与熔体，最终形成了A型花岗岩的母岩浆。