Geochemistry and Geochronology of A-Type Intermediate-Felsic Rocks in NW Himalaya, Pakistan: Implications for Petrogenesis and Tectonic Evolution of Northern Gondwana

Abstract

Igneous intrusives in northern Pakistan can provide valuable insights into pre-Himalayan metaluminous to peraluminous magmatism along the northern boundary of the supercontinent Gondwana and its potential tectonic significance. This study generates new geochronologic, petrographic and geochemical data for intermediate (monzonite, syenite, and foid syenite) and felsic (granite and quartz monzonite) rocks within the NW Himalayan region of Pakistan. Both the intermediate and felsic rocks have values of A/NK > 1.1, implying a metaluminous to peraluminous composition, and are distinguished by high FeO_T/(MgO + FeO_T) (0.81–1.0), high 10,000 × Ga/Al ratio (2.1–5.1), elevated Nb + Zr + Y + Ce contents (122–1,204 ppm), and negative anomalies of P and Ti, consistent with aluminous A-type magmatic affinity. These rocks are classified as A₁-type, which is linked to anorogenic intraplate extensional setting. Both rock groups yield high calculated average whole-rock Zr saturation temperatures (i.e., T_Zr; 790–823°C), which suggests formation from high-temperature magmas. Whole-rock geochemistry, including variable (⁸⁷Sr/⁸⁶Sr)_i values (0.7034–0.7086), positive εNd(t) (+0.1 to +3.9), high Pb isotopic values (that is, (²⁰⁶Pb/²⁰⁴Pb)_i = 18.68 to 19.31, (²⁰⁷Pb/²⁰⁴Pb)_i = 15.64 to 15.74, and (²⁰⁸Pb/²⁰⁴Pb)_i = 38.93 to 39.78), and variation in zircon εHf(t) values (+0.8 to +7.0), indicates diverse magma sources for the intermediate-felsic rocks and provide evidence of partial melting of metasomatized lithospheric mantle, producing a primary magma of foid to quartz syenitic composition. Subsequently, this magma was responsible for the partial melting of the overlying juvenile crust, producing granitic, quartz monzonitic and monzonitic magmas. During the magma evolution process of these rocks, this process was primarily determined by partial melting that followed fractionation of K-feldspar, ilmenite and apatite. T_DM2 ages indicate that the parent materials of intermediate-felsic rocks were generated during the Mesoproterozoic-Neoproterozoic. LA-ICP–MS U-Pb dating of magmatic zircons documents their formation in the Late Paleozoic at ∼278-268 Ma. The intermediate-felsic rocks are correlatable with alkaline igneous rocks of the Peshawar Plain, which record the breakup of the supercontinent Gondwana and the subsequent opening of Neo-Tethys during a Late Paleozoic rifting event. The nepheline syenite records a younger episode during the Cenozoic (37 Ma), corresponding to the collision of the Indian-Eurasian tectonic plates.