Origin And Timing Of Alkaline Magmatism And Associated REE-Th Mineralization From Kohistan Island Arc, Pakistan: Implication From U-Th-Pb Dating, Mineral Geochemistry And Sr-Nd-Hf Isotopic Systematics

Alkaline igneous-related rare earth element(REE)deposits constitute a major global economic resource of REE.However,our understanding of their geodynamic settings and magmatic-hydrothermal processes remains uncertain.Here,we present an investigation of petrography,geochronology,mineral chemistry,bulk rock and Sr-Nd-Hf isotopes of newly discovered REE mineralization in the alkaline syenite,pegmatite,and adakitic granite from the Kohistan Batholith(KB)in the Kohistan Island Arc(KIA),Pakistan to understand the petrogenesis and tectonic evolution.The pegmatites contain abundant chevkinite and monazite(up to 10-35 vol.%),which are the main REE-bearing phases and formed from crystallization of a REE-oversaturated evolved melt.U-Th-Pb isotopic dating of zircon from the syenite yields the emplacement age of 31.2±0.3 Ma,which closely matches the ages of zircon and monazite from the pegmatite 30.1±0.3 Ma and 30.9±0.3 Ma,respectively.Zircons from the syenite and the pegmatite have similar?_Hf(t)values,ranging from+0.2 to+2.5,indicating that the syenite and the pegmatite are co-magmatic in origin.Zircons show the evolutional trend in trace element compositions from the syenite to the pegmatite,marked by increases in REE,Hf,Y,U,and volatiles.The calculated Ce^*,Ce^4+/Ce³⁺andfO₂ yielded higher values for the pegmatite than those for the syenite,indicating that REE mineralization in the pegmatite occurred under more oxidizing conditions.Zircons from the syenite have higher Ti contents than those from the pegmatite.The calculated zircon saturation temperatures based on the thermometric model of yield higher values 626-700? for the syenitic magma than those for the pegmatitic magma 496-645?.The chemical compositions of titanite also shows the evolutionary trend from the syenite to the pegmatite,marked by increases in REE,Sr,Ga,Eu*,Fe₂O₃/Al₂O₃and decreases in CaO and Ce^*.The magnitude of the Eu*and Ga and Fe₂O₃/Al₂O₃ ratios in titanites are mainly depends on the redox state of the magma.The late titanite in the pegmatite yielded higher values Ga and Fe₂O₃/Al₂O₃than those for the syenite further conforming that,REE mineralization in pegmatites was occurred in more oxidizing conditions.Moreover,the Ga and Fe₂O₃/Al₂O₃ratios positively correlate with REE content in the titanite and calculated oxygen fugacity of zircons.The sharp compositional changes of endmember of biotite to phlogopite and aegirine-augite to aegirine from the syenite to the pegmatite are marked by the increasing trend of F,SiO₂,MgO,X_Mg,Na₂O,FeO_t,and decreasing TiO₂,Al₂O₃,CaO and Mn O,which suggests that the sudden change in the crystallizing environment.The change of biotite to phlogopite coupled with increase of anomalous F content suggest that the late-stage magma became volatile and water saturated.This inference is further supported by high S content in pegmatite monazite and higher model abundance of amphiboles and calcite in the pegmatites.The evolution of aegirines from syenite to pegmatite marked increasing of aegirine component(Na and Fe³⁺),strong depletion of diopside(Ca,Mg)with an insignificant increase of jade(Na,Al)suggest that pegmatitic stage magma become more alkaline than syenitic stage.This point is further supported by the lack of quartz in pegmatites as compared to syenites.The multiple generations of accessory minerals and ferromagnesium silicates reveal prolonged fractional crystallization processes of the alkaline magma,which were responsible for the REE enrichment in the residual pegmatitic melt of magmatic origin.Based on these studies,we can delineate two distinct magmatic events:a)an early syenitic stage represented early phases of magma evolution which is characterized by low volatiles,water,oxygen fugacity;b)the late pegmatitic stage magma which is characterized by distinctively higher volatile contents,water,oxygen fugacity and lower temperature conditions.The late magmatic event is responsible for the observed REE mineralization in the KIA.Zircon U-Pb dating indicates that the adakitic granite crystallized at ca.33.2 Ma,similar to or slightly earlier than the crystallization ages of the syenite(31.2 Ma)and the pegmatite(30.9 Ma).The adakitic granite is characterized by high Al₂O₃(13.05-15.46wt.%),Sr(683-1161 ppm),and low Y(4.6-8.9 ppm)contents,and thus high Sr/Y ratios(123-230).The adakitic granite has very low contents of MgO(0.18-0.27 wt.%)and Ni(0.47-0.75 ppm),indicating that its parental magma was likely derived from partial melting of thickened lower crust.The syenitic rocks are potassic to ultra-potassic shoshonitic in nature by high K₂O contents(8.7-10.3 wt.%)and K₂O/Na₂O ratios(2.2-3.15),strong enrichment in LILEs,LREEs,and depletion in HFSEs,P,Ti,and Y.The geochemical and Sr-Nd-Hf isotopic signatures of the syenite indicate that its parental magma was likely derived by partial melting of the metasomatized oceanic crust underneath the lower crust.The low Ba/La,Hf/Sm and high Th/Yb ratios of the shoshonitic syenite strongly imply that metasomatism of oceanic crust was majorly controlled by the pelagic and carbonate sediments of Tethyan Ocean.The peralkaline magma was likely formed by the partial melting of metasomatized oceanic crust induced by slab break-off following the earlier adakitic granite at the post-collisional stage between the KIA with the Indian plate.The post-collisional alkaline rocks are widely distributed in Tethyan orogenic belt.These observations indicate that these rocks may be good potential for REE mineralization.