Shallow Continental Lithospheric Mantle Heterogeneity — Petrological Constraints

Abstract

Variations in the mineralogical composition of the shallow continental lithospheric mantle (SCLM) can be studied directly using two kinds of ultramafic material: (a) mantle xenoliths entrained in mafic alkaline magmas and (b) tectonically emplaced ultramafic massifs in orogenic belts. Both types of mantle-derived material have been well studied in several regions of western and central Europe. Data from these two separate but complementary sources can be used to deduce the major processes which formed and modified the SCLM. In Phanerozoic regions of Europe, most ultramafic xenoliths are spinel peridotites, which contain 45–95% olivine, 10–30% orthopyroxene, 0–18% clinopyroxene and ca. 3% spinel. The bulk of the SCLM is composed of spinel lherzolite, with only minor amounts of more refractory harzburgite. Other less common mineralogical varieties also occur, e.g. pyroxenites and hornblendites, which sometimes appear as veins or layers that cut spinel peridotite. Pyroxenites and hornblendites are most probably formed as cumulates from mafic magmas that have traversed the SCLM. All these ultramafic xenoliths are derived from shallow depths (<80km), i.e. above the garnet-spinel transition, as very few of the peridotites contain garnet. Plagioclase-bearing peridotite xenoliths, which would represent mantle <27km deep, are also absent. Mantle xenolith suites from Phanerozoic Europe show broad mineralogical, chemical and textural similarities to continental spinel peridotites worldwide. Significant differences from this norm indicate that fundamentally different processes have occurred. Field relations between mantle rock-types can be observed in ultramafic massifs such as those found in the Pyrenees, Betics and Alps. Lithologies are identical to those found as ultramafic xenoliths, but massifs reveal structural complexities such as pyroxenite layering, harzburgite bands and several generations of cross-cutting mafic and ultramafic dykes or veins. Major and trace element variations in xenoliths and in peridotite massifs are similar. The main lithospheric mantle process revealed by petrological and geochemical analysis of massifs and xenoliths is depletion by removal of a basaltic melt, forming residual mantle. Subsequent enrichment has occurred by passage of a variety of silicate melts, which gave rise to layers of anhydrous pyroxenite and veins containing amphibole or phlogopite.