Abstract 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 eHf(t) values of +1.2 to +8, δ18O of 6.4 to 7.4‰ and bulk rock eNd(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 isotopes (eNd(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 eNd(t) (−1.4 to +1.3), zircon eHf(t) (+2.8 to +10.4) and δ18O (4.7–7.3‰) values, but radiogenic bulk rock Pb isotopes. The geochemical and isotopic signatures 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 distinctive geochemical features compared to I-type granitoids, including enrichment in Nb-Ta, high bulk rock eNd(t) (+3.3 to +3.9) and zircon eHf(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 derived from a sub-continental lithospheric mantle, but which interacted 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 decompression melting of SCLM. Rising mantle-derived magmas assimilated middle-upper crustal rocks. Fractionating mantle-derived magmas and contamination with crustal components produced evolved gabbro-diorites and I-type granites. In contrast, asthenospheric upwelling during the Paleocene provided heat for melting and interaction with SCLM to generate the precursor melts to the A-type granites.

中文翻译：

---

锆石 U Pb，对伊朗西北部 A 型和 I 型花岗岩和辉长闪长岩年龄和成因的地球化学和同位素约束

摘要 晚白垩世I型花岗岩和辉长闪长岩以及古新世A型花岗岩侵入伊朗西北大陆地壳。锆石的 SIMS 和 LA-ICPMS U-Pb 分析得出 I 型花岗岩和辉长闪长岩的年龄为 100-92 Ma（晚白垩世），A 型花岗岩的年龄为 61-63 Ma（古新世）。来自伊朗西北部的晚白垩世辉长闪长岩（包括镁铁质微粒飞地；MME）显示出不同的演化特征。它们在 N-MORB 归一化的微量元素蜘蛛图上显示出 Nb 和 Ta 的消耗，并且具有高的 Th/Yb 比率，表明它们的前体岩浆是在与俯冲相关的环境中产生的。辉长闪长岩的锆石 eHf(t) 值为 +1.2 至 +8，δ18O 值为 6.4 至 7.4‰，大块岩石 eNd(t) 值为 -1.4 至 ~ +4.9。地球化学和同位素数据证明了次大陆岩石圈地幔 (SCLM) 的熔化产生了具有放射性 Nd 同位素 (eNd(t) = ~ +4.9) 和高 Nb/Ta 和 Zr/Hf 比率的近原始辉长岩，类似于地幔熔融(Nb/Ta ~ 17 和 Zr/Hf ~ 38)。这些镁铁质熔体经过进一步分馏并与地壳熔体混合，生成晚白垩世演化出的辉长闪长岩。I 型花岗岩的地球化学数据表明 Nb-Ta 负异常和正异常以及轻稀土元素的富集。这些岩石是过铝质的，具有可变的大块岩石 eNd(t)（-1.4 至 +1.3）、锆石 eHf(t)（+2.8 至 +10.4）和 δ18O (4.7-7.3‰) 值，但具有放射性大块岩石 Pb 同位素. 这些花岗岩的地球化学和同位素特征表明，地幔衍生的基性岩浆（类似于近原始的 Oshnavieh 辉长岩）与中上地壳通过同化分晶化 (AFC) 相互作用产生晚白垩世 I 型花岗岩。与I型花岗岩相比，古新世A型花岗岩具有独特的地球化学特征，包括Nb-Ta富集、高块岩eNd(t)（+3.3至+3.9）和锆石eHf（t）（+5.1 - +9.9）值。碱性花岗岩是铁质的；它们具有低 MgO、CaO、Sr、Ba 和 Eu 浓度，以及高总 Fe2O3、K2O、Na2O、Al2O3、Ga、Zr、Nb-Ta、Th 和稀土元素 (REE) 丰度以及 Ga/Al 比率。这些岩石可能与来自次大陆岩石圈地幔的熔体的分馏有关，但与软流圈衍生的熔体相互作用。我们认为俯冲开始和由此产生的板坯回滚导致上覆伊朗板块的极端伸展，诱发地幔楔中的对流并导致 SCLM 减压熔化。上升的幔源岩浆同化了中上地壳岩石。地幔源岩浆的分馏和地壳成分的污染产生了演化出的辉长闪长岩和 I 型花岗岩。相比之下，古新世软流圈上涌为熔化和与 SCLM 相互作用提供了热量，从而产生了 A 型花岗岩的前体熔体。地幔源岩浆的分馏和地壳成分的污染产生了演化出的辉长闪长岩和 I 型花岗岩。相比之下，古新世软流圈上涌为熔化和与 SCLM 相互作用提供了热量，从而产生了 A 型花岗岩的前体熔体。地幔源岩浆的分馏和地壳成分的污染产生了演化出的辉长闪长岩和 I 型花岗岩。相比之下，古新世软流圈上涌为熔化和与 SCLM 相互作用提供了热量，从而产生了 A 型花岗岩的前体熔体。