Geochronological and geochemical studies of adakites from Tethyan Belt, Western Pakistan: A clue to geodynamics and Cu-Au mineralization

ABSTRACT

Large porphyry Cu-Au deposits are associated with Early Miocene intrusive rocks in Tethyan belt, discovered along Chagai magmatic arc in Western Pakistan, adjacent to Southeast Iran. Two types of rocks were discriminated as granodiorite and monzodiorite from Saindak area. The granodiorites are associated with regional large Cu-Au mineralization, while the monzodiorites are mostly ore-barren. LA-ICP-MS U-Pb dating yielded weighted mean ²⁰⁶Pb/²³⁸U ages of 22.16–24.2 Ma for granodiorites and ca. 22.6 Ma for a monzodiorite. These intrusive rocks in the Saindak area are all calc-alkaline rocks, with enriched LILEs and depleted HFSEs, and without Eu negative anomalies. The felsic granodiorites are characterized by high Sr contents and Sr/Y ratios, with intermediate to high (La/Yb)_N ratios that identify them as typical high-silica adakites. In contrast, the basaltic-andesitic monzodiorites are just normal arc-related rocks, showing less fractionated REE patterns than the granodiorites, with systematically lower LREE and higher MREE and HREE. Low K₂O/Na₂O ratios and decoupled Sr/Y-(La/Yb)_N ratios indicate the characteristics of slab-derived adakites for the granodiorites, most likely originated through partial melting of the subducted Neotethys oceanic crust beneath Eurasian continent followed by subsequent mantle interaction. The almost simultaneously melting of subcontinental lithospheric mantle induced by dehydration of Neotethys plate gave rise to the formation of the barren monzodiorites. These intrusive rocks in the Saindak area are characterized by similar Sr-Nd-Pb isotopes with high ⁸⁷Sr/⁸⁶Sr_i ratios, slightly negative εNd(t) values and radiogenic Pb isotopes, plotting in the field between the MORB and EM-II mantle endmembers or the Average Cadomian Lower Crust, suggesting subducting sediments or old continental crustal materials have contributed into the compositions of these rocks by source enrichment or crustal contamination. The slab-melting derived adakite is favourable for regional massive Cu-Au mineralization in the Saindak area.

Graphical Abstract

KEYWORDS:

• Adakite
• slab melting
• mantle melting
• Saindak
• porphyry Cu-Au deposit
• early miocene
• neotethys plate subduction

Highlights

• Saindak granodiorites and monzodiorites are young in age of 22-24 Ma.

• Granodiorites are distinguished as high-silica adakite, while monzodiorites are normal arc rocks.

• Granodiorites are formed by partial melting of young Neotethys oceanic slab, while monzodiorites are form by sub-arc mantle melting.

• Slab-derived granodiorites favor Cu-Au mineralization, while arc mantle-derived monzodiorites are barren.

Acknowledgments

This study is supported by the National Key R&D Program of China (No. 2016YFC0600404), National Natural Science Foundation of China (No. 41673040) and CAS-TWAS President’s Fellowship. We expressed our great thanks to Dr Badar Muneer, Mr Muhammad Ramzan and Mr Faiz for the fieldwork at Saindak area, Balochistan and thanks to Manager Saindak Mining who gave us residence in Saindak colony rest house throughout the fieldwork. Dr Robert J. Stern Editor-in-Chief and two anonymous reviewers are greatly appreciated for constructive review comments, which have significantly improved the manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

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Funding

This work was supported by the National Natural Science Foundation of China [41673040];National Key R&D Program of China [2016YFC0600404].