Geochemical and isotopic data of meta-gabbroic dikes in Alaşehir area in the Menderes Massif, Western Turkey: Evidence for the Early Cambrian back-arc rifting?
Stratigraphic feature of undated garnet-bearing meta-gabbroic dikes cross-cutting basement rocks of meta-pelitic successions in the Alaşehir areas (Central Menderes Massif - CMM) are still in debate and are distinct from the eclogitic meta-gabbroic dikes, garnet amphibolites and biotite rich meta-gabbroic dikes cross-cutting Neoproterozoic basement rocks (meta-tonalites/meta-granodiorites and meta-granites) that crop out in the Birgi, Tire and Çine areas of the CMM. The garnet-bearing meta-gabbro dikes in the Alaşehir areas have low Nb (0.6 to 4.9 ppm), low Th (0.2 to 0.5 ppm), low Zr (10.1 to 54.6 ppm), low to moderate Y (8.4 to 15.1 ppm), Ti (2518 t 6175 ppm), Zr/Yb (13.33 to 41.31 ), Ti/Zr (57.05 to 346.86), Y/Zr (0.23 to 0.98) ratios. They are highly depleted in LREE and display flat patterns in MREE and HREE compared to the eclogitic meta-gabbroic dikes, garnet amphibolites, biotite rich meta-gabbroic dikes of the Late Neoproterozoic basement rocks. Nb, Zr, Ti contents with Nd/Sm and La/Yb ratios compared to La/Yb and Eu/Eu* indicate distinct enrichment of the garnet-bearing meta-gabbroic dikes (Alaşehir areas) relative to the eclogitic meta-gabbroic dikes, garnet amphibolites and biotite rich gabbroic dikes (Birgi, Tire and Çine areas). The low Nb, low Th, low Zr contents, and high Ti/Zr and low Zr/Yb ratios in the garnet-bearing meta-gabbroic dikes (Akşehir areas) compared to the eclogitic meta-gabbroic dikes, biotite rich meta-gabbroic dikes indicates that they may have been derived from melting of mafic lower crustal sources. Variable ɛ Nd(540) (-2.9 to +1.9) and 147Sm/144Nd ratios (0.1320 to 0.2277) of the garnet-bearing meta-gabbroic dikes in Alaşehir areas indicate extensive mixing of a mafic lower crustal source with depleted mantle during their formation. Apparent crustal residence Nd model ages (TDM) determined using the two-stage model of Liew and Hofmann (1988) range from 1.08 - 1.47 Ga, also indicating mixing of older sources with depleted mantle. We suggest that Andean-type subduction was followed by lithospheric thinning of the continental crust, resulting in an initial stage of back-arc basin formation in the peri-Gondwanan crust during the Late Neoproterozoic. The biotite-rich meta-gabbroic dikes may have been generated during the initial stage of this rifting, whereas garnet-bearing meta-gabbroic dikes (Alaşehir areas) may have been generated later during the mature stage of continental rifting in the Early Cambrian. Similar tectonic scenarios have been suggested for other peri-Gondwanan terranes in Europe, the Middle East, and eastern Asia
