Origin of rare earth element variations in clinopyroxene from plutonic and associated volcanic rocks from the Foulde basin, Northern Kedougou inlier, Senegal, West Africa

摘要

The Paleoproterozoic basin of Foulde, in the northern part of the Kedougou inlier (eastern Senegal), comprises both volcanic and plutonic suites belonging to the same K-rich calc-alkaline series. Plutonic and volcanic rocks are coeval and they show strong compositional similarities in major and trace elements including rare earths (REE). Clinopyroxene is ubiquitous in the mafic rocks and has the same major-element (except CaO) and trace-element (except LREE, Th and U) contents in both Suites. The clinopyroxene found in basalts and basaltic andesites has convex upward chondrite-normalized REE patterns, with a LREE depletion [(Ce/Yb)(N) = 0.64-0.67)]. Clinopyroxene from gabbros has chondrite-normalized REE patterns characterized by a LREE enrichment with (Ce/Yb)(N) = 1.86-1.66. These differences are related to the emplacement mode of the two magmatic suites, i.e. to the cooling rate of the magma. In volcanic rocks, very rapid cooling allows clinopyroxene to retain its primary magmatic composition. In plutonic rocks the lower cooling rate allows late-crystallization and subsolidus processes, which modify the early trace-element composition. The magmatic processes affecting the monzogabbros are of two types: (i) reaction between the main igneous phases (clinopyroxene and plagioclase) and an interstitial melt crystallizing quartz and K-feldspar, and (ii) exsolutions of amphibole, ilmenite and orthopyroxene in the clinopyroxene. The core of monzogabbros clinopyroxene that shows no exsolution has a higher CaO content than that of clinopyroxene in the volcanic rocks. We propose that exsolution of Ca-and-LREE depleted orthopyroxene in the inner rim of the clinopyroxene in monzogabbros induces an increase of the Ca content of its core and may explain its compositional difference (Ca and LREE) with the clinopyroxene in volcanic rocks.