Geochemistry and geochronology of high-K metaluminous (I-type) Cambrian Guwahati granites from Shillong Plateau, Northeast India: Insight into petrogenesis involving peritectic assemblage entrainment (PAE) model

The peritectic assemblage entrainment (PAE) model effectively explains variations in Fe, Mg, Ti, and Ca, as well as positive correlations of Ti and Ca with increased maficity [molar (Fe + Mg)] in mafic I-type granitic melts. This study analyzes the whole-rock elemental geochemistry and zircon U-Pb-Hf isotopes of newly identified Ordovician to Cambrian mafic-enriched I-type granites from the Guwahati region of the Shillong Plateau, northeastern India. The key geochemical features include positive correlations between Ti and maficity, along with V, Ca, and P, while molar A/CNK and maficity demonstrate a negative correlation. These patterns suggest the granites formed from the incongruent melting of hydrous ferromagnesian minerals in the crust, incorporating a peritectic assemblage of plagioclase, FeTi oxides, and clinopyroxene. The UPb zircon ages of 523 ± 4.94–473 ± 3.5 Ma demonstrate the emplacement age of the Guwahati granites. Zircon Hf isotopic analysis indicates a two-stage model age (T_DM2) of 2095–2036 Ma with negative εHf(t) values between −10.66 and − 8.47. This suggests a significant contribution from Paleoproterozoic continental crust in the formation of the 473 ± 3.5 Ma Ordovician granitic magma. In comparison, the Cambrian granites, dated around 523 to 502 Ma with weighted mean ²⁰⁶Pb/²³⁸U age of 513 ± 36 Ma, exhibit a slightly higher T_DM2 of 2098–2092 Ma and show exclusively negative εHf(t) values, ranging from −9.95 to −9.63. These findings indicate a consistent involvement of ancient crustal sources in both granite types. Consequently, it can be concluded that the Ordovician and Cambrian Guwahati granites share notable similarities in their model ages and εHf(t) values, reflecting common geological processes underlying their formation. Overall, the Guwahati granites (523–473 Ma) are linked to Pan-African tectonic activity associated with Ordovician-Cambrian arc magmatism and the tectono-thermal events following the collision of India, Australia, and Antarctica during the assembly of the Eastern Gondwana Landmasses. This process involved the melting of Paleoproterozoic basement crustal rocks.