Mineral composition in cognate inclusions in Late Miocene-Early Pliocene potassic lamprophyres with affinities to lamproites from the Denizli region, Western Anatolia, Turkey: Implications for uppermost mantle processes in a back-arc setting

Abstract

Mineral assemblages of both cognate inclusions and host-lamprophyres with lamproitic affinity in Kocapinar district (Denizli), Western Anatolia, consist of phlogopitic mica, diopsidic clinopyroxene, K-feldspar, apatite, opaque and carbonates (calcite-dolomite). Two distinct types of cognate inclusions have been defined in host lamprophyres: clinopyroxene-rich (CCI) and clinopyroxene-phlogopite-rich (CPCI). Whole-rock compositions of inclusions show a near-primitive nature with high MgO (7.56-15.1wt.%), Cr (195-2270ppm), Ni (213-335ppm) contents and a potassic character [K 2O (2.0-2.8wt.%)>Na 2O (0.4-2.2wt.%)]. Data imply that the inclusions crystallized from magmas formed by melting of phlogopite-bearing pyroxenites in a peridotitic mantle source, however, the presence of reverse-zoning in clinopyroxenes with salitic Fe-rich green cores in host-lamprophyres and CPCI inclusions suggest that the Denizli lavas represent mixtures of distinct (probably ultrapotassic and alkali basaltic) magmas. Estimated geobarometric constraints inferred from clinopyroxene compositions in CCI and CPCI inclusions indicate moderate pressures of pyroxene crystallization (ranging between 1.7 and 2.2GPa and corresponding to 53-70km depths) under low pressure magma fractionation. Results reveal that i) the origin of Denizli lamprophyres with transitional (between arc-type and intra-plate-type) geochemical signatures is consistent with a shallow level mantle petrogenesis, rather than a deep-seated origin related to mantle convection, ii) the source was a highly refractory and metasomatized peridotitic mantle present at the base of the lower crust, iii) metasomatic agents that affected the mantle lithosphere beneath Denizli region are distinct from those beneath other western Anatolia orogenic centers, and iv) the transitional character of Kocapinar (Denizli) lamprophyric rocks were probably formed as a result of either underplating or contamination of asthenospheric magma at the base of the mantle lithosphere, or assimilation of delaminated continental edge-lithospheric mantle via ascending asthenosphere. Denizli lamprophyres are interpreted to have formed during the formation of a basin/graben structure in the Latest Miocene-Early Pliocene, just after Late Miocene exhumation of Menderes massif. © 2012 Elsevier B.V.