Petrology, Geochemistry, and Mineralogy of the Early Cretaceous Evolved N-MORB from Sites 765 and 766, Eastern Indian Ocean

Abstract

The Early Cretaceous basaltic rocks obtained from Sites 765 and 766 in the eastern Indian Ocean floor were mostly iron-rich normal mid-ocean ridge basalts (N-MORB), which were derived from a depleted mantle source having strongly light rare earth element (LREE)-depleted rare-earth patterns and a high titanium/zirconium (Ti/Zr) ratio. Basaltic rocks in the upper part of the Site 765 basement section include megacrysts and gabbroic fragments of widely varying mineral chemistry. These megacrysts range from An 90 plagioclase, including highly magnesian basaltic glass coexisting with augite of Mg# (= 100 Mg/[Fe+Mg]) at 85, to An 50 plagioclase coexisting with hypersthene. This varying mineralogy of megacrysts and gabbroic fragments indicates that a considerable degree of fractional crystallization took place in the magma chamber. The unusual negative correlation between incompatible elements (e.g., TiO 2 ) and FeO*/MgO observed among Site 765 basement basalts and fresh volcanic glasses suggest source-mantle heterogeneity in terms of FeO*/MgO. Strontium isotope ratios ( 87 Sr/ 86 Sr) of the basaltic rocks from both sites are between 0.7027 and 0.7033 and are comparable to those of mid-Indian Ocean ridge basalts (MIORB).The basalt pebbles encountered in the sedimentary section may have been transported from the basement highs nearer the Australian continent and include basaltic compositions ranging from primitive N-MORBs to evolved enriched(E)-MORBs. Their mantle source was not as depleted as that of the basement basalts. These rocks may be the products of earlier volcanism that took place during the rifting of the Australian continent.

Delineating the Exmouth mantle plume (NW Australia) from denudation and magmatic addition estimates

“…Thus, it seems unlikely that significant amounts of eclogite melting (>15%) were involved in the Exmouth mantle plume, even when considering higher values for the thermal expansion coefficient (a) (Schutt and Lesher, 2006). Limited sampled magmatism at ODP Site 766 provides evidence for low-pressure fractionation of plagioclase and clinopyroxene (cumulate gabbro; Ludden and Dionne, 1992), consistent with the underplating theory (Farnetani et al, 1996), though a normal oceanic source cannot be completely ruled out (Ishiwatari, 1992). Other dredge samples from the region are highly weathered (Sayers et al, 2002) and unsuitable for detailed geochemistry interpretation.…”

Oceanic plateau accretion inferred from Late Paleozoic greenstones in the Jurassic Tamba accretionary complex, southwest Japan

“…The absence of a negative Nb anomaly argues against IAT identification. MORB with negative correlation between incompatible elements and FeO*/MgO have been reported from the eastern Indian Ocean by Ishiwatari (1992), who concluded that the unusual negative correlation can be attributed to heterogeneity in the source mantle in terms of the FeO*/MgO ratio. Thus, it is concluded here that N‐type greenstones originated in the ocean floor basalt that constituted the normal oceanic crust (N‐MORB) mixed blocks.…”

Geology and petrology of the plutonic complexes in the Wadi Fizh area: Multiple magmatic events and segment structure in the northern Oman ophiolite

“…For a comparison, fields of oceanic gabbros, MORB (phenocrysts), Troodos, IAT (phenocrysts of island arc tholeiite) are also shown. Data sources: oceanic gabbros: Beard [1986] and Juteau and Maury [1999], Troodos: Thy [1987], MORB and IAT: Ishiwatari [1992], Arc Type I and II and tholeiitic layered intrusions: Beard [1986], Haymiliyah, Salahi, Rustaq and Ragmi: Lachize et al [1996]. All rocks plot on a similar trend within the oceanic gabbro and MORB phenocryst fields except for those of the Fizh-South complex.…”