Mineralogy, whole-rock and Sr–Nd isotope geochemistry of mafic microgranular enclaves in Cretaceous Dagbasi granitoids, Eastern Pontides, NE Turkey: Evidence of magma mixing, mingling and chemical equilibration

Abstract

Rocks of the Late Cretaceous Dagbasi Pluton (88–83 Ma), located in the eastern Pontides, include mafic microgranular enclaves (MMEs) ranging from a few centimetres to metres in size, and from ellipsoidal to ovoid in shape. The MMEs are composed of gabbroic diorite, diorite and tonalite, whereas the felsic host rocks comprise mainly tonalite, granodiorite and monzogranite based on both mineralogical and chemical compositions. MMEs are characterized by a fine-grained, equigranular and hypidiomorphic texture. The common texture of felsic host rocks is equigranular and also reveals some special types of microscopic textures, e.g., oscillatory-zoned plagioclase, poikilitic K-feldspar, small lath-shaped plagioclase in large plagioclase, blade-shaped biotite, acicular apatite, spike zones in plagioclase and spongy-cellular plagioclase textures and rounded plagioclase megacrysts in MMEs. Compositions of plagioclases (An 33–An 60), hornblendes (Mg#=0.77–1.0) and biotites (Mg#=0.61–0.63) of MMEs are slightly distinct or similar to those of host rocks (An 12–57; hbl Mg#=0.63–1.0; Bi Mg#=0.50–0.69), which suggest partial to complete equilibration during mafic–felsic magma interactions. The felsic host rocks have SiO 2 between 60 and 76 wt% and display low to slightly medium-K tholeiitic to calc-alkaline and peraluminous to slightly metaluminous characteristics. Chondrite-normalized rare-earth element (REE) patterns are fractionated (La cn/Lu cn=1.5–7.3) with pronounced negative Eu anomalies (Eu/Eu*=0.46–1.1). Initial ε Nd(i) values vary between −3.1 and 1.6, initial 87Sr/ 86Sr values between 0.7056 and 0.7067. Compared with the host rocks, the MMEs are characterized by relatively high Mg-number of 22–52, low contents of SiO 2 (53–63 wt%), low ASI (0.7–1.1) and low to medium-K tholeiitic to calc-alkaline, metaluminous to peraluminous composition. Chondrite-normalized REE patterns are relatively flat [(La/Yb) cn=1.4–3.9; (Tb/Yb) cn=0.9–1.5] and show small negative Eu anomalies (Eu/Eu*=0.63–1.01). Isotope signatures of these rocks ( 87Sr/ 86Sr (i)=0.7054–0.7055; ε Nd(i)=–1.0 to 1.9) are largely similar to the host rocks. Gabbroic diorite enclaves have relatively low contents of SiO 2, ASI; high Mg#, CaO, Al 2O 3, TiO 2, P 2O 5, Sr and Nb concentrations compared to dioritic and tonalitic enclaves. The geochemical and isotopic similarities between the MMEs and their host rocks indicate that the enclaves are of mixed origin and are most probably formed by the interaction between the lower crust- and mantle-derived magmas. All the geochemical data suggest that a basic magma derived from an enriched subcontinental lithospheric mantle, interacted with a crustal melt that originated from dehydration melting of the mafic lower crust at deep crustal levels. The existence of compositional and textural disequilibrium and the nature of chemical and isotopic variation in these rock types indicate that magma mixing/mingling between an evolved mafic and a granitic magma was involved in their genesis. Microgranular enclaves are thus interpreted to be globules of a more mafic magma probably from an enriched lithospheric mantle source. Al-in-amphibole estimates the pluton emplacement at ca. 0.3–3.8 kbar, and therefore, magma mixing and mingling must have occurred at 3.8 kbar or below this level.