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

IF 3.6 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
Amjad Hussain , Kui-Dong Zhao , Mohammad Arif , Bin Fu , Martin R. Palmer , Wei Chen , Shao-Yong Jiang , Delores M. Robinson , Hafiz U. Rehman
{"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 ,&nbsp;Kui-Dong Zhao ,&nbsp;Mohammad Arif ,&nbsp;Bin Fu ,&nbsp;Martin R. Palmer ,&nbsp;Wei Chen ,&nbsp;Shao-Yong Jiang ,&nbsp;Delores M. Robinson ,&nbsp;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}
引用次数: 0

Abstract

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.
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
碰撞造山带内从埃达克质花岗岩到过碱性超典型岩石的演替为研究区域构造演化提供了有价值的线索。本文研究了巴基斯坦北部Kohistan岛弧(KIA)的Kohistan岩基中新发现的埃达克质花岗岩和过碱性超古生代正长岩的详细岩石学、锆石U-Pb年龄、全岩地球化学数据和同位素数据。锆石U-Pb测年表明,埃达质花岗岩的侵位时间为~ 33.6 Ma,略早于正长岩的~ 31.2 Ma。阿达基花岗岩具有高SiO2、高K2O、高Na2O、高Al2O3、高Sr、高Sr/Y的特征,低MgO、低Ni、低Cr的特征,可能反映了加厚下地壳中水溶基性岩石的部分熔融作用。阿达基花岗岩的εHf(t)值为正(平均±3.6),δ18Ozir值为幔状(平均±5.3‰),表明岩浆来源于加厚的下地壳基性源,俯冲沉积物和上地壳物质对岩浆的贡献不大。正长岩为钾质-超古生代/钾玄生质,K2O含量高(8.7 ~ 10.3 wt%), K2O/Na2O比值高(2.2 ~ 3.2),LILE和LREE富集,HFSE贫。地球化学和Sr-Nd-Hf-O同位素数据表明,正长岩可能来源于下地壳下交代岩石圈地幔的部分熔融。低Ba/La(平均值为7.4)和Hf/Sm(平均值为0.06)比值结合同位素模拟表明,俯冲的远洋海相沉积物对KIA超古典纪过碱性岩石的地幔源贡献为10 - 20%,这与早期特提斯洋壳的俯冲有关。这些地球化学结果提供了证据,表明在印度-起亚碰撞(~ 45 Ma)后的~ 10 Ma,西喜马拉雅发生了板块断裂,并导致了阿达基岩和超古生代岩石的侵位,类似于在特提斯造山带其他地方观察到的碰撞后碱性岩浆作用。这些同时期但岩石成因不同的岩浆突出了特提斯造山带碰撞后演化过程中地壳和地幔非均质性的相互作用。此外,该研究完善了我们对KIA碰撞后碱性岩浆活动的认识,并为全球其他类似的汇聚造山带提供了更广泛的地球动力学模型。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
求助全文
约1分钟内获得全文 求助全文
来源期刊
Chemical Geology
Chemical Geology 地学-地球化学与地球物理
CiteScore
7.20
自引率
10.30%
发文量
374
审稿时长
3.6 months
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.
×
引用
GB/T 7714-2015
复制
MLA
复制
APA
复制
导出至
BibTeX EndNote RefMan NoteFirst NoteExpress
×
提示
您的信息不完整,为了账户安全,请先补充。
现在去补充
×
提示
您因"违规操作"
具体请查看互助需知
我知道了
×
提示
确定
请完成安全验证×
copy
已复制链接
快去分享给好友吧!
我知道了
右上角分享
点击右上角分享
0
联系我们:info@booksci.cn Book学术提供免费学术资源搜索服务,方便国内外学者检索中英文文献。致力于提供最便捷和优质的服务体验。 Copyright © 2023 布克学术 All rights reserved.
京ICP备2023020795号-1
ghs 京公网安备 11010802042870号
Book学术文献互助
Book学术文献互助群
群 号:604180095
Book学术官方微信