Neoproterozoic felsic magmatism in southern Kerala, India: The building blocks of Gondwana

Abstract

The assembly of Gondwana supercontinent involved multiple stages of continent convergence during the Neoproterozoic including arc magmatism and final collisional metamorphism. Peninsular India occupies a key position in the Gondwana supercontinent assembly where widespread Neoproterozoic magmatic records are preserved. Here, we investigate representative felsic magmatic intrusions from the Achankovil Suture Zone that marks the boundary between the Madurai and Trivandrum crustal blocks. We present petrological, geochemical, and zircon U‐Pb and Lu‐Hf data from the Chengannur (CH) and Pathanapuram (PT) plutons. Petrographic studies on syenogranites from the CH pluton indicate I‐type features, and feldspar thermometry shows ultrahigh temperatures (~1,000°C) corresponding to similar temperatures reported for the surrounding metamorphic rocks. Geochemically, the PT suite displays island arc affinity, whereas the CH suite shows adakitic, oxidized A‐2 type features. In tectonic discrimination plots, the CH samples correspond to volcanic arc granite, whereas the PT samples traverse syn‐collisional granite and volcanic arc granite zones, indicating a transitional feature. Trace element relationships suggest magma fractionation dominantly controlled by plagioclase, K‐feldspar, amphibole, and biotite. Magmatic zircon grains from the CH rocks show upper intercept ages in the range of 783 to 819 Ma and lower intercept ages in the range of 519 to 545 Ma, with cumulative age peaks at 766 and 529 Ma. The combined age population of monzogabbro, granodiorite, and granite from the PT pluton shows distinct peaks at 1,021, 950, and 537 Ma. Zircon grains from the CH pluton show dominantly negative εHf (t) values in the range of −5.8 to −13.4 and yield Hf depleted model age (TDM) ranging from 1,541 to 1857 Ma and Hf crustal model ages ( ) of 2,029 to 2510 Ma. In contrast, zircon grains from the PT pluton show dominantly positive εHf (t) values (2.9 to 9.8) with depleted model ages (TDM) in the range of 1,145 to 1434 Ma and Hf crustal model ages ( ) in 1,164 to1,686 Ma. The zircon U‐Pb and Lu‐Hf data clearly indicate that the timing of magmatism and magma sources is different for the two plutons, although both witnessed a common metamorphic event during late Neoproterozoic–Cambrian. The PT pluton was emplaced during the early Neoproterozoic with magma source involving mostly juvenile or depleted mantle components, as also attested to by the geochemical signature of arc affinity. In contrast, the CH pluton formed later during mid‐Neoproterozoic with magma from deeper source and dominated by recycled Paleoproterozoic material, which possibly involved depleted mantle components, both associated with an earlier subduction regime. The results reported in our study correlate with those from the arc magmatic rocks in the southern part of the Madurai Block as well as from the crustal blocks in the adjacent terrane of Sri Lanka. The extensive Neoproterozoic arc magmatism in a subduction‐related setting involving both juvenile additions and recycled older components represents the building blocks of Gondwana.