Insight into crustal contamination and hydrothermal alteration of the Panjal Traps (Kashmir) from O-isotopes

Whole rock O-isotopes of the Panjal Traps are presented in order to assess the influence of crustal contamination and hydrothermal alteration in their genesis. The basalts from the eastern Kashmir Valley and Lidder Valley have enriched δ18OVSMOW values (9.0‰ to 12.0‰). The basalt with the lowest δ18OVSMOW values (9.0‰ and 9.2‰) were likely affected by deuteric alteration but their values could be close to the original melt composition as the rocks do not show trace element (Th/NbPM ≈0.8; Nb/U ≈ 50; Th/La ≈0.1) or isotopic evidence of crustal contamination (87Sr/86Sri = 0.7043 to 0.7045; εNd(t) = +1.1 to +1.3). The δ18OVSMOW (> 12‰) values and Nd isotopes (εNd(t) < −8.6) of the silicic Panjal Traps are consistent with derivation from continental crust. The remaining mafic rocks have enriched Sr-Nd-O isotopic values that indicate crustal (10–30%) contamination (εNd(t) = −1.9 to −6.1; 87Sr/86Sri = 0.7051 to 0.7087; δ18O = 10.1‰ to 12.0‰) with the upper flows exhibiting further enrichment by hydrothermal alteration. The basalts from the Pir Panjal Range, western Kashmir Valley, have variable Nd isotopic values (εNd(t) = −6.8 to +4.3) and the lowest δ18OVSMOW values (6.8‰ to 7.9‰) of the study. The results demonstrate that the rocks from the Pir Panjal Range preserved not only differences in radiogenic isotopes but also the O-isotopes as well. The change in the Nd and O isotopes of the basalt from Guryal Ravine and Lidder Valley to the more depleted values of the Pir Panjal Range is likely due to a transition from a chondritic mantle source to a depleted mantle source as the continental rift evolved to a sea-floor spreading environment.

本研究所呈现的帕尔安山脉全岩氧同位素数据，旨在评估壳源污染和水热蚀变对其成因的影响。来自东部克什米尔谷和利迪河谷的玄武岩呈现出增高的δ^18OVSMOW值（9.0‰至12.0‰）。具有最低δ^18OVSMOW值（9.0‰和9.2‰）的玄武岩可能受到了氘化蚀变的影响，但其值可能接近原始熔融组成，因为岩石未显示出微量元素（Th/NbPM ≈0.8；Nb/U ≈ 50；Th/La ≈0.1）或同位素壳源污染的证据（87Sr/86Sri = 0.7043至0.7045；εNd(t) = +1.1至+1.3）。硅质帕尔安山脉的δ^18OVSMOW (> 12‰)值和钕同位素（εNd(t) < −8.6）与源自大陆地壳的来源一致。剩余的基性岩石具有增高的Sr-Nd-O同位素值，表明存在壳源（10–30%）的污染（εNd(t) = −1.9至−6.1；87Sr/86Sri = 0.7051至0.7087；δ^18O = 10.1‰至12.0‰），而上部岩流由于水热蚀变而进一步富集。皮尔帕尔安山脉西部克什米尔谷的玄武岩具有可变的钕同位素值（εNd(t) = −6.8至+4.3）和研究中最低的δ^18OVSMOW值（6.8‰至7.9‰）。结果表明，皮尔帕尔安山脉的岩石不仅保留了放射成因同位素的不同，同时也保留了氧同位素。从古里雅峡谷和利迪河谷至皮尔帕尔安山脉更贫的值的变化，可能是由于随着大陆裂谷演变为洋中脊扩张环境，从球粒陨石地幔源向贫化地幔源的过渡所致。