[S-MP24] Supercontinents and Crustal Evolution

[SMP24-P02] Evidence of Paleoproterozoic rift-to-drift transition in eastern margin of Singhbhum Craton, India from U-Pb zircon, Sr-Nd isotopic and geochemical study of dismembered gabbro-wehrlite of Bangriposi

*Tushar Mouli Chakraborti^1,2, Kosuke Kimura^1,2, Arijit Ray³, Ramananda Chakrabarti⁴, Gautam Kumar Deb³ (1.Earth and Planetary System Science program, Graduate School of Advanced Science and Engineering, Hiroshima University, Japan, 2.Hiroshima Institute of Plate Convergence Region Research (HiPeR), Hiroshima University, Japan, 3.Department of Geology, Presidency University, Kolkata, India, 4.Centre for Earth Sciences, Indian Institute of Science, Bangalore, India)

Keywords:Layered gabbro, Wehrlite, Cumulate dunite, Rift-to-drift, Ophioplite, Columbia supercontinent

Dismembered modally layered gabbro and serpentinized wehrlite are found emplaced within multiply deformed metapelites of the North Singhbhum Mobile Belt assemblage, Singhbhum Craton, India. Zircon grains from the gabbro unit analyzed by LA-ICP MS yields an average U-Pb crystallization age of 1881.1 ± 8.4 Ma. The undeformed, cumulate gabbroic body exhibits excellent modal and cryptic layering, generated by the interplay of ad cumulate and textural coarsening growth process. It is moderately evolved with Mg number (100Mg/Mg+Fe) 48-63, low Alkali and TiO₂, high Al₂O₃; enrichment of Pb, Ba, Rb with depletion of U, Th, Zr and unfractionated rare earth patterns with positive Eu anomaly. The serpentinized wehrlite shows evidence of cumulate nature of the protolith and extensive post-crystallization metasomatism. Mineral chemistry, whole-rock geochemistry and clinopyroxene trace elements reveals progressive fluid-rock interaction from very high-temperature to low-temperature regimes. Calculated melts in equilibrium with the gabbroic rocks display enrichment in lighter elements similar to EMORBs and resembles the estimated average composition of the metasomatic fluid of the wehrlite rocks. The gabbros have initial ⁸⁷Sr/⁸⁶Sr 0.70600-0.71099, initial ¹⁴³Nd/¹⁴⁴Nd 0.510130- 0.510294, and E_Nd (t) 1.8 to -1.4. The serpentinized wehrlite displays extreme enrichment with initial ⁸⁷Sr/⁸⁶Sr 0.71045- 0.72640, initial ¹⁴³Nd/¹⁴⁴Nd 0.509120- 0.509706, and almost crust-like E_Nd (t) of -9.7 to -21.2. The cumulate nature of the mafic-ultramafic rocks with other petrographical evidences imply formation in a static magma chamber. Dismembered nature, occurrence within an orogenic belt assemblage and temporal considerations suggest emplacement by obduction and affinity towards oceanic magma chambers. This is also reflected in mineralogy, whole-rock geochemistry and trace element compositions of modeled equilibrium melts which resemble rift volcanics and are similar to mafic dykes of continental margin ophiolites. The isotopic character of the gabbros can be explained by 30%-40% mixing between a depleted and a subduction-influenced enriched domain in the Sub-Continental Lithospheric Mantle beneath Singhbhum Craton. The isotopic signature of the wehrlite has been perturbed by the metasomatic event and reflects the enriched nature of the fluid phase. The occurrence of the mafic-ultramafic rocks within the shallow-deep marine, intra-continental rift basinal assemblage of Chaibasa formation, shows that rift-to-drift transition took place during Paleoproterozoic and incipient oceanic crust was formed. A 1.86 Ga orogenic event marked by intrusion of anorogenic granites caused closure of this basin and obduction of these fragments onto the orogenic belt assemblage. This result represents the first such discovery in Singhbhum Craton and can be correlated Paleoproterozoic ophiolites of south India. The formation and emplacement of these oceanic crustal rocks fits with the rift-dominated tectonics of that era and the widespread continental amalgamation and ophiolite emplacement which took place in Columbia supercontinent during Paleoproterozoic.