Melt-fluid infiltration in Archean suprasubduction zone mantle wedge: Evidence from geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes from Wynad, southern India

Abstract

The southern Peninsular India preserves a number of Archean cratonic nuclei among which the Coorg Block is considered to represent a Mesoarchean microcontinent. Here we investigate the southern margin of this block where remnants of rare ultramafic rocks in association with arc magmatic rocks and accreted metasediments occur in the Wynad region. We present petrology, mineral chemistry, whole rock geochemistry and zircon U–Pb and Lu–Hf data from a suite of meta ultramafics and associated diorite, orthopyroxene-bearing felsic charnockite, and quartz-mica schist. The peak P–T conditions of metamorphism of these rocks are estimated at 720–840°C and 6.5–8.7kbar. The geochemical features of the ultramafic suite provide evidence for melting and metasomatism of lithospheric mantle in a suprasubduction zone (SSZ) setting with the involvement of both silicate melts and slab-dehydrated fluids. The parent melts were generated at varying depth levels extending from spinel to garnet stability fields. A subduction-modified, LREE replenished, metasomatically enriched mantle source endorses arc-like chemical signatures in the melts that were acquired through interaction between the depleted sub-arc lithospheric mantle and subduction components.We report the common presence of magmatic zircons in the meta ultramafic rocks with crystallization ages of 3.3–3.0Ga. Some of these rocks also carry another younger group of magmatic zircons with crystallization ages down to ca. 2.5Ga. Zircons in the orthopyroxene-bearing felsic charnockite also formed at 3.2–3.0Ga whereas those in the metadiorite crystallized at around 2.6–2.5Ga. The detrital zircons in the quartz mica schist show upper intercept age of 3.3Ga, and lower intercept at 2.5Ga. The Mesoarchean ages from the Wynad suite are consistent with the ages reported from the Coorg Block, whereas the Neoarchean ages correspond with those in the Nilgiri Block to the south. The Lu–Hf data from zircon grains in the various samples show both negative and positive εHf(t) values ranging from −18.23 to 13.98. The data yield Hf depleted mantle model ages (TDM) of 2562–3837Ma and Hf crustal residence model ages (TDMC) ranging from 2576Ma to 4009Ma, indicating magma derivation from both juvenile and reworked components of Neohadean to early-mid Archean age. Our results reveal a suprasubduction zone mantle wedge that has been repeatedly replenished through multiple events of subduction zone magmatism in the Mesoarchean and Neoarchean.