Amjad Hussain , Kui-Dong Zhao , Mohammad Arif , Bin Fu , Martin R. Palmer , Wei Chen , Shao-Yong Jiang , Delores M. Robinson , Hafiz U. Rehman
{"title":"巴基斯坦北部Kohistan岛弧新发现的超古典纪和艾达克质岩石的锆石U-Pb年龄、地球化学和Sr-Nd-Hf-O同位素数据:对岩石成因和碰撞后碱性岩浆作用的制约","authors":"Amjad Hussain , Kui-Dong Zhao , Mohammad Arif , Bin Fu , Martin R. Palmer , Wei Chen , Shao-Yong Jiang , Delores M. Robinson , Hafiz U. Rehman","doi":"10.1016/j.chemgeo.2025.122989","DOIUrl":null,"url":null,"abstract":"<div><div>The succession from adakitic granite to peralkaline ultrapotassic rocks within collisional orogens provides valuable insights into the regional tectonic evolution. This study presents detailed petrography, zircon U-Pb ages, whole-rock geochemical data, and isotopic data from newly discovered adakitic granite and peralkaline ultrapotassic syenite rocks from the Kohistan Batholith in the Kohistan Island Arc (KIA), northern Pakistan. Zircon U-Pb dating indicates that the adakitic granite was emplaced at ∼33.6 Ma, slightly earlier than the syenite at ∼31.2 Ma. The adakitic granite is characterized by high contents of SiO<sub>2,</sub> K<sub>2</sub>O, Na<sub>2</sub>O, Al<sub>2</sub>O<sub>3,</sub> Sr, and Sr/Y ratios, as well as low MgO, Ni, and Cr, which likely reflects partial melting of water-fluxed mafic rocks in the thickened lower crust. The positive ε<sub>Hf</sub>(t) (avg. +3.6) and mantle-like δ<sup>18</sup>O<sub>zir</sub> (avg. +5.3 ‰) values of the adakitic granite suggest that the magma was derived from a thickened lower crustal mafic source, with no significant contribution from subducted sediments or upper crustal materials. The syenite is potassic to ultrapotassic/shoshonitic, characterized by high K<sub>2</sub>O contents (8.7–10.3 wt%) and K<sub>2</sub>O/Na<sub>2</sub>O ratios (2.2–3.2), and exhibits enrichment in LILE and LREE while being depleted in HFSE. Geochemical and Sr–Nd–Hf–O isotopic data indicate that the syenite likely derived from partial melting of the metasomatized lithospheric mantle beneath the thickened lower crust. The low Ba/La (avg. 7.4) and Hf/Sm (avg. 0.06) ratios, combined with isotopic modeling, reveal a 10–20 % contribution of subducted pelagic marine sediments to the mantle source of ultrapotassic peralkaline rocks in the KIA, which is linked to subduction of the earlier Tethyan Ocean crust. These geochemical results provide evidence that slab break-off occurred in the western Himalaya ∼10 Ma after the India–KIA collision at ∼45 Ma, and resulted in the emplacement of the adakitic and ultrapotassic rocks, similar to post-collisional alkaline magmatism observed elsewhere in the Tethys Orogenic Belt. These coeval but petrogenetically distinct magmas highlight the interplay of crustal and mantle heterogeneity during post-collisional evolution across the Tethyan Orogen. In addition, this study refines our understanding of the post-collisional alkaline magmatism in the KIA and provides a broader geodynamic model applicable to other similar convergent orogenic belts worldwide.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"693 ","pages":"Article 122989"},"PeriodicalIF":3.6000,"publicationDate":"2025-07-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":"{\"title\":\"Zircon U-Pb ages, geochemical and Sr-Nd-Hf-O isotopic data of newly discovered ultrapotassic and adakitic rocks in the Kohistan Island Arc, North Pakistan: Constraints on petrogenesis and post-collisional alkaline magmatism\",\"authors\":\"Amjad Hussain , Kui-Dong Zhao , Mohammad Arif , Bin Fu , Martin R. Palmer , Wei Chen , Shao-Yong Jiang , Delores M. Robinson , Hafiz U. Rehman\",\"doi\":\"10.1016/j.chemgeo.2025.122989\",\"DOIUrl\":null,\"url\":null,\"abstract\":\"<div><div>The succession from adakitic granite to peralkaline ultrapotassic rocks within collisional orogens provides valuable insights into the regional tectonic evolution. This study presents detailed petrography, zircon U-Pb ages, whole-rock geochemical data, and isotopic data from newly discovered adakitic granite and peralkaline ultrapotassic syenite rocks from the Kohistan Batholith in the Kohistan Island Arc (KIA), northern Pakistan. Zircon U-Pb dating indicates that the adakitic granite was emplaced at ∼33.6 Ma, slightly earlier than the syenite at ∼31.2 Ma. The adakitic granite is characterized by high contents of SiO<sub>2,</sub> K<sub>2</sub>O, Na<sub>2</sub>O, Al<sub>2</sub>O<sub>3,</sub> Sr, and Sr/Y ratios, as well as low MgO, Ni, and Cr, which likely reflects partial melting of water-fluxed mafic rocks in the thickened lower crust. The positive ε<sub>Hf</sub>(t) (avg. +3.6) and mantle-like δ<sup>18</sup>O<sub>zir</sub> (avg. +5.3 ‰) values of the adakitic granite suggest that the magma was derived from a thickened lower crustal mafic source, with no significant contribution from subducted sediments or upper crustal materials. The syenite is potassic to ultrapotassic/shoshonitic, characterized by high K<sub>2</sub>O contents (8.7–10.3 wt%) and K<sub>2</sub>O/Na<sub>2</sub>O ratios (2.2–3.2), and exhibits enrichment in LILE and LREE while being depleted in HFSE. Geochemical and Sr–Nd–Hf–O isotopic data indicate that the syenite likely derived from partial melting of the metasomatized lithospheric mantle beneath the thickened lower crust. The low Ba/La (avg. 7.4) and Hf/Sm (avg. 0.06) ratios, combined with isotopic modeling, reveal a 10–20 % contribution of subducted pelagic marine sediments to the mantle source of ultrapotassic peralkaline rocks in the KIA, which is linked to subduction of the earlier Tethyan Ocean crust. These geochemical results provide evidence that slab break-off occurred in the western Himalaya ∼10 Ma after the India–KIA collision at ∼45 Ma, and resulted in the emplacement of the adakitic and ultrapotassic rocks, similar to post-collisional alkaline magmatism observed elsewhere in the Tethys Orogenic Belt. These coeval but petrogenetically distinct magmas highlight the interplay of crustal and mantle heterogeneity during post-collisional evolution across the Tethyan Orogen. In addition, this study refines our understanding of the post-collisional alkaline magmatism in the KIA and provides a broader geodynamic model applicable to other similar convergent orogenic belts worldwide.</div></div>\",\"PeriodicalId\":9847,\"journal\":{\"name\":\"Chemical Geology\",\"volume\":\"693 \",\"pages\":\"Article 122989\"},\"PeriodicalIF\":3.6000,\"publicationDate\":\"2025-07-29\",\"publicationTypes\":\"Journal Article\",\"fieldsOfStudy\":null,\"isOpenAccess\":false,\"openAccessPdf\":\"\",\"citationCount\":\"0\",\"resultStr\":null,\"platform\":\"Semanticscholar\",\"paperid\":null,\"PeriodicalName\":\"Chemical Geology\",\"FirstCategoryId\":\"89\",\"ListUrlMain\":\"https://www.sciencedirect.com/science/article/pii/S0009254125003791\",\"RegionNum\":2,\"RegionCategory\":\"地球科学\",\"ArticlePicture\":[],\"TitleCN\":null,\"AbstractTextCN\":null,\"PMCID\":null,\"EPubDate\":\"\",\"PubModel\":\"\",\"JCR\":\"Q1\",\"JCRName\":\"GEOCHEMISTRY & GEOPHYSICS\",\"Score\":null,\"Total\":0}","platform":"Semanticscholar","paperid":null,"PeriodicalName":"Chemical Geology","FirstCategoryId":"89","ListUrlMain":"https://www.sciencedirect.com/science/article/pii/S0009254125003791","RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q1","JCRName":"GEOCHEMISTRY & GEOPHYSICS","Score":null,"Total":0}
Zircon U-Pb ages, geochemical and Sr-Nd-Hf-O isotopic data of newly discovered ultrapotassic and adakitic rocks in the Kohistan Island Arc, North Pakistan: Constraints on petrogenesis and post-collisional alkaline magmatism
The succession from adakitic granite to peralkaline ultrapotassic rocks within collisional orogens provides valuable insights into the regional tectonic evolution. This study presents detailed petrography, zircon U-Pb ages, whole-rock geochemical data, and isotopic data from newly discovered adakitic granite and peralkaline ultrapotassic syenite rocks from the Kohistan Batholith in the Kohistan Island Arc (KIA), northern Pakistan. Zircon U-Pb dating indicates that the adakitic granite was emplaced at ∼33.6 Ma, slightly earlier than the syenite at ∼31.2 Ma. The adakitic granite is characterized by high contents of SiO2, K2O, Na2O, Al2O3, Sr, and Sr/Y ratios, as well as low MgO, Ni, and Cr, which likely reflects partial melting of water-fluxed mafic rocks in the thickened lower crust. The positive εHf(t) (avg. +3.6) and mantle-like δ18Ozir (avg. +5.3 ‰) values of the adakitic granite suggest that the magma was derived from a thickened lower crustal mafic source, with no significant contribution from subducted sediments or upper crustal materials. The syenite is potassic to ultrapotassic/shoshonitic, characterized by high K2O contents (8.7–10.3 wt%) and K2O/Na2O ratios (2.2–3.2), and exhibits enrichment in LILE and LREE while being depleted in HFSE. Geochemical and Sr–Nd–Hf–O isotopic data indicate that the syenite likely derived from partial melting of the metasomatized lithospheric mantle beneath the thickened lower crust. The low Ba/La (avg. 7.4) and Hf/Sm (avg. 0.06) ratios, combined with isotopic modeling, reveal a 10–20 % contribution of subducted pelagic marine sediments to the mantle source of ultrapotassic peralkaline rocks in the KIA, which is linked to subduction of the earlier Tethyan Ocean crust. These geochemical results provide evidence that slab break-off occurred in the western Himalaya ∼10 Ma after the India–KIA collision at ∼45 Ma, and resulted in the emplacement of the adakitic and ultrapotassic rocks, similar to post-collisional alkaline magmatism observed elsewhere in the Tethys Orogenic Belt. These coeval but petrogenetically distinct magmas highlight the interplay of crustal and mantle heterogeneity during post-collisional evolution across the Tethyan Orogen. In addition, this study refines our understanding of the post-collisional alkaline magmatism in the KIA and provides a broader geodynamic model applicable to other similar convergent orogenic belts worldwide.
期刊介绍:
Chemical Geology is an international journal that publishes original research papers on isotopic and elemental geochemistry, geochronology and cosmochemistry.
The Journal focuses on chemical processes in igneous, metamorphic, and sedimentary petrology, low- and high-temperature aqueous solutions, biogeochemistry, the environment and cosmochemistry.
Papers that are field, experimentally, or computationally based are appropriate if they are of broad international interest. The Journal generally does not publish papers that are primarily of regional or local interest, or which are primarily focused on remediation and applied geochemistry.
The Journal also welcomes innovative papers dealing with significant analytical advances that are of wide interest in the community and extend significantly beyond the scope of what would be included in the methods section of a standard research paper.