Moho transition zone in the Cretaceous Andaman ophiolite, India: A passage from the mantle to the crust

Highlights

• Moho transition zone documents melt–mantle interaction and fractional crystallization.

• Olivine-rich troctolites record impregnation of MORB melt into replacive dunite.

• The composition of the impregnated melt is indistinguishable with the gabbroic rocks.

• Clinopyroxenites formed from a melt genetically linked with the replacive dunite.

Abstract

We examined the composition and lithological variability from a road section in south-Andaman which represents a pathway to the crustal section of the Cretaceous Andaman ophiolite. Like other well-studied ophiolites worldwide this transition zone is marked by association of olivine-rich troctolite, wehrlite, pyroxenite and gabbroic rocks. The mineral chemical variations document the evolution of this zone by melt–mantle interaction and fractional crystallization. Petrographic evidence suggests that water was introduced during the evolution of this transition zone. The olivine-rich troctolites record impregnation of MORB melt into a residual olivine-rich lithology (replacive dunite) that formed by an earlier episode of melt–peridotite interaction at a slow spreading ridge. The clinopyroxenites indicate formation from an extreme clinopyroxene saturated melt that might be genetically linked with the formation of olivine-rich protolith of the troctolitic rocks prior to melt impregnation. The wehrlite crystallized from the melt residual after the formation of clinopyroxenite. The composition of the impregnating melt that transformed the replacive dunite to olivine-rich troctolite is identical to the gabbroic rocks. We conclude that the association of these rock types from south-Andaman provides us with a snapshot of the switch over of geodynamic setting of the Andaman ophiolite (MOR to arc) as preserved presently between north-Andaman in the north and Rutland Island in the south.

Introduction

The Moho transition zone (MTZ) in an ophiolite sequence represents a zone of magmatic exchange between mantle and the oceanic crust. Compositionally this zone demarcates a gradational zone from the uppermost residual mantle peridotites from which melt is extracted to the base of continuous gabbroic rocks, an unequivocal crystallization product from magma that initiates the formation of oceanic crust. This gradational zone signifies the location where crystallization takes place prior to formation of the oceanic crust. However, crystallization may start in melt channels deeper in the mantle as observed in abyssal peridotites (e.g. Dantas et al., 2007, Dick and Natland, 1996, Seyler et al., 2001), in ophiolites (e.g., Ceuleneer et al., 1996, Python and Ceuleneer, 2003, Varfalvy et al., 1997) and as inferred from MORB composition (e.g., Grove et al., 1992).

The MTZ may vary in thickness from few meters to several hundreds of meters. Thick MTZ characterized by vertical mantle flow occurs above Maqsad diapir of Oman, whereas thin MTZ correlates with off-axis mantle flow away from the diapir (Rabinowicz et al., 1987). Nevertheless, this cannot be systematically generalized because there are places where thick dunitic horizons are observed with no evidence of vertical mantle flow. This is the case, among others, of the Wadi Thuqbah area of Oman (Negishi et al., 2013), of the Trinity ophiolite (Ceuleneer and le Sueur, 2008), and of Newfoundland ophiolites (Suhr et al., 2003). Lithologically the MTZ is dominated by olivine-rich rocks (mainly dunitic) which are commonly impregnated, and host discontinuous bands or lenses of wehrlites, troctolites and gabbros. The MTZ was initially thought to be the bottommost part of the crustal section mainly because of the notion that the lithological variations observed in this zone may be explained by mineral fractionation from a mid-ocean ridge basaltic magma. Accordingly, a cumulus origin of the MTZ was proposed (Elthon, 1979, O'Hara, 1965). Subsequent work, however, on samples collected during the Ocean Drilling Program (and submersible dive operations) (Dick et al., 2010, Sanfilippo et al., 2013, Suhr et al., 2008) and studies carried out on several ophiolite sections (Benoit et al., 1996, Ceuleneer, 1991, Clenet et al., 2010, Jousselin and Nicolas, 2000, Koepke et al., 2009, Koga et al., 2001, Python and Ceuleneer, 2003, Renna and Tribuzio, 2011, Sanfilippo and Tribuzio, 2013) propose a primarily residual mantle origin for the host dunites (replacive), formed largely by melt–mantle reaction. Significantly, however, the origin of this dunitic transition zone (DTZ) is still debated and recently, Abily and Ceuleneer (2013) from their study on the Oman ophiolite argued that the dunitic rocks in the mantle–crust transition zone, although dominantly residual in origin in their lowermost parts, grade into cumulate dunite at the top.

This paper will focus on the nature and origin of the MTZ in the Andaman ophiolite, India, based on detailed field, petrographic, mineral chemical and petrological studies. Although the lithological diversity of the mantle peridotites from this ophiolite has been described in many recent papers (e.g., needs references here) the identification of MTZ and its characterization with reference to melt percolation through this zone have not yet been attempted. The main objective of this paper is to determine the sequential formation of various rock types, explaining their association from a mantle-borne basaltic melt continuously evolving through this zone. Characterisation of various MTZ rock types in this section of oceanic lithosphere gives us an insight to the processes of melt extraction (likely episodic) from the shallow mantle below the paleo-Indian Ocean and migration of the evolving melt through the transition zone to form the crustal sequence. This study records a progressive geodynamic evolution of the Cretaceous Andaman ophiolite which had its origin at mid-ocean ridge and was later modified by subduction.