Pervasive mantle plume head heterogeneity: Evidence from the late Cretaceous Caribbean-Colombian oceanic plateau

In SW Colombia picritic pillow lavas and tuffs, as well as breccias composed of picritic clasts, occur interspersed with basalts of the Central Cordillera and represent accreted portions of the ∼90 Ma Colombian/Caribbean oceanic plateau (CCOP). We present new geochemical data for these picrites and high-MgO basalts from SW Colombia, along with new data from Deep Sea Drilling Project Leg 15 drill sites. The 40Ar/39Ar ages for the CCOP in the Central Colombian Cordillera range from 87 to 93 Ma. Both SW Colombia picrites and Leg 15 basalts are compositionally diverse and range from reasonably enriched ((La/Nd)n > 1 and (εNd)i +8.0). Nb/Y and Zr/Y systematics suggest that the depleted component is not depleted MORB mantle, but is an intrinsic part of the plume. The bulk of the CCOP compositions can be explained by mixing between this depleted mantle and a HIMU component. However, radiogenic isotope systematics indicate the presence of an EM2 (or possibly EM1) component within the plume. Mantle melt modeling suggests that the enriched magma types are the product of deeper, small degree melting of a pervasively heterogeneous plume comprising a refractory matrix with enriched streaks/blobs, whereas shallower, more extensive melting, results in the formation of relatively depleted magmas

Country rock contamination of marginal mafic granulites bordering the Nain Plutonic Suite : implications for mobilization of Sr during high-grade contact metamorphism

The marginal mafic granulites that locally border the Nain Plutonic Suite (NPS) have a range of initial Nd-isotope ratios that overlap with that of the NPS anorthosites and associated Nain dykes. The similarity in Nd-isotope data suggests that gneissic Archaean country rocks have contaminated all the anorthosites, marginal mafic granulites, and dykes. Sr-isotope data for the mafic granulites and dykes support a country rock contamination scenario but preclude wholesale assimilation of rocks such as the host Archaean tonalite gneisses as the sole contaminant. Initial epsilonSr values of +10 to +403 and +0.9 to +242 for the mafic granulites and dykes, respectively, are significantly higher than values for NPS country rocks examined thus far. The elevated initial eSr values are therefore interpreted to result from the introduction of radiogenic Sr into the granulites and dykes via Sr-rich fluids, generated by the breakdown of Rb-rich mineral phases such as biotite in the country rocks during NPS

Geochemistry and petrogenesis of late Archaean Aravalli volcanics, basement enclaves and granitoids, Rajasthan

Major, trace and rare earth element data are presented for the basal volcanics of the Aravalli supracrustal sequence, for the mafic enclaves within the Banded Gneiss Complex (BGC) adjacent to the supracrustals, and for granitoids within the BGC and their petrogenesis discussed with regard to the problematic relationship between basement and cover. In terms of major elements the Aravalli lavas range from magnesian (22% MgO) komatiites (or picrites) to Fe-rich tholeftes, but differ from a typical Archaean greenstone volcanic sequence in that their trace element patterns are relatively enriched in incompatible and light rare earth elements. Moreover the range of patterns observed requires a number of distinct mantle sources as well as different degrees of melting. There are two distinct types of mafic enclave within the BGC. The larger lenses correspond quite well with the range of variation seen in the Aravalli sequence, and could be feeders; the other type, close to the BGC/Aravalli junction, has more alkalic characteristics (high Nb/Zr) and could represent rift volcanics, or tectonically accreted fragments of ocean islands. The data suggest that the Aravalli sequence is not an accreted oceanic terrane, but are more consistent with interaction of a deep mantle plume with rather mature sub-continental lithosphere. The early Archaean age of parts of the BGC, the chemistry of the granitoids and the alkaline character of the enclaves are also broadly compatible with this model: the character of the Aravalli sequence is intermediate between that of an Archaean greenstone belt and a Phanerozoic flood basalt province

Geochemistry and petrogenesis of Garhwal volcanics: implications for evolution of the North Indian lithosphere

Middle Proterozoic magmatism in the Garhwal Himalayas consists of an extensive sequence of mafic volcanics plus associated mafic dyke sheets. Geochemical data reveal that the volcanic suite is tholeiitic, with the dyke sheets showing considerably more Fe-enrichment than the lava flows. All the rocks are distinctly enriched in incompatible elements and light REE relative to primordial mantle abundances, but have an equally distinct "continental" signature reflected in marked negative Nb, Sr, P and Ti anomalies in their mantle-normalised spidergrams. Although such geochemical characteristics are commonly thought to indicate contamination of the magmas with a continental crust component, the evidence is strongly in favour of the compositional characteristics being inherited from their mantle source. Other Proterozoic volcanics in northern India have closely similar geochemical features, as have other classic occurrences of younger continental flood basalt suites in adjacent areas of Gondwanaland. These geochemical characters are probably imposed upon the subcontinental lithosphere at the time of continent formation but, in the case of younger CFB suites, this may have been modified to a greater or lesser extent by subsequent additions of "plume" mantle material