Turkish Journal of Earth SciencesFractional Crystallization and Magma Mingling/Mixing Processes in the Monzonitic Association in the SW Part of the Composite Yozgat Batholith (Şefaatli-Yerköy, SW Yozgat)
Cumhuriyet Üniversitesi, Jeoloji Müh. Bölümü, 58140 Sivas-TURKEY
Abstract: The Yozgat batholith takes place in the passive margin of the Anatolide-Pontide convergence system. It includes; syn-collisional S-type, two-mica granites; post-collisional I-type, calcalkaline monzonitic association, and post-collisional, M-type, tholeiitic gabbroic/dioritic association. Among these associations, the monzonitic association is also subdivided into five mapable units such as Cankılı monzogabbro/monzodiorite, Akçakoyunlu quartz monzodiorite, Adatepe quartz monzonite, Yassiagil monzogranite and Karakaya monzogranite. All these subunits, except the Karakaya monzogranite, include K-feldspar megacrysts. These five subunits represent some evidence of fractional crystallization (FC) and magma mingling/mixing processes by means of field, mineralogical-petrographical and geochemical characteristics. There is a good zonation in the field that the Cankılı monzogabbro/monzodiorite forms the core, and Karakaya monzogranite forms the rim. Mafic mineral assemblage of the Cankılı monzogabbro/monzodiorite, first product of FC process, comprizes clinopyroxene + amphibole (hornblende + hastingsite) + biotite. Those of Akçakoyunlu and Adatepe subunits, crystallized just after the Cankılı by FC during cooling of magma, are composed of amphibole (hornblende + hastingsite) + clinopyroxene + biotite. As for the mafic minerals of the Yassiagil and Karakaya subunits, solidified as final FC products comprise amphibole (hornblende + hastingsite) + biotite ± clinopyroxene, and biotite + amphibole (hornblende + hastingsite) ± clinopyroxene, respectively. The major element oxides such as tFe 2 O 3 , MnO, MgO, CaO, TiO 2 and P 2 O 5 apparently decrease with the increasing silica content, whereas K 2 O content increases. The trace elements like Co, Cu, Zn, Rb and Ga determine good FC trend, on the contrary Cr, Pb, Nb, Y, Zr and Th elements exhibit slightly FC trend versus silica content. Variations in the Sr and Ba elements versus silica also describe recognizable FC trend from Cankılı to Karakaya units, although, some secondary mobilizations in the contents of these trace elements, particularly depletions in some Cankılı rock samples do not fit well this trend. The most reliable variograms indicating FC trend are those of K/Rb versus Rb, K/Ba versus Ba and of K/Rb versus K/Ba. Some mafic microgranular magmatic enclaves (MME), commonly found in the field, are considered to be evidences of the magma mingling process. There are also some microscopic textures (antirapakivi, poikılıtic/oikocrystic feldspars, small lath shaped plagioclase within large plagioclase, spike zones in plagioclase and spongy cellular dissolution/melting textures in plagioclase) indicating the magma mixing process.
Key Words: Composite Yozgat batholith, fractional crystallization, magma mingling/magma mixing, S type, I type, Mtype assosiations.
Turkish J. Earth Sci., 7, (1998), 215-230.
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Other articles published in the same issue: Turkish J. Earth Sci.,vol.7,iss.3.