Basalt magma genesis and fractionation in collision- and extension-related provinces: A comparison between eastern, central and western Anatolia


Coban H.

EARTH-SCIENCE REVIEWS, cilt.80, ss.219-238, 2007 (SCI İndekslerine Giren Dergi) identifier identifier

Özet

Experimental studies of synthetic and natural basalt systems suggest that conditions of magma genesis and fractionation depend fundamentally on mantle temperatures and lithospheric stress fields. In general, compressional settings are more conducive to polybaric fractionation than extensional settings and in this regard, the Anatolian magmatic province offers a natural laboratory for comparing near-coeval basalt eruptions as a function of regional tectonics - compressional (collision-related) regimes dominating in eastern Anatolia and extensional tectonics characterizing a western province related to Aegean Sea opening. Projection of Plio-Quaternary basalt normative compositions from the Western Anatolia Extensional Province (WAEP), the Central. Anatolian 'Ova' Province (CAOP), and Eastern Anatolia Compressional Province (EACP) are projected onto Ol-Ne-Cpx and Pl-Cpx-Ol planes in the simplified basalt system (Ne-Cpx-Ol-Qz), each showing distinctive liquid lines of descent. WAEP basalts are mostly constrained by low-pressure (< 0.5 GPa) cotectics while CAOP and EACP compositions conform to moderate and/or high-pressure (0.8-3.0 CTa) cotectics. Overall, a quasi-linear shift from moderate and/or high-pressure to low-pressure equilibria matches the westward transgression from compressional cast Anatolia to the extensional west Anatolian-Aegean region. Comparison of their respective primary (mantle-equilibrated) magmas -simulated by normalizing their compositions to MgO - 15 wt.% (Mg-15)-With parameterized anhydrous and H2O-undersaturated experimental melts suggests they segregated from spinel- to gamet-lherzolite mantle facies at pressures between c. 2 and 3 GPa (c. 70- 100 kin depth) under H2O-undersaturated conditions. Interpolated potential temperatures (T-p) and lithospheric stretching factors (ss) range as follows: (1) eastern Anatolian basalts associated with the Arabian foreland show T-p varying between 1250 and 1400 degrees C (except for the Karacalidag alkali basalts, south of the Bitlis-Zagros fracture zone, for which T-p ranges up to 1450 degrees C), for ss values of 1,2-1.8. T-p values for central Anatolia (e.g. Sivas) range between 1300 and 1375 degrees C (except for Karapinar, Egrikuyu and Hasandag, which show < 1150 degrees C), and 0 values of 1.3-1.4. For western Anatolian basalts, T-p range mostly between 1250 and 1330 degrees C, except for a single value for Canakkale of 1400 degrees C and Kula sample showing Tr < 1200 degrees C, and ss values of 1.3-2.0. Variation of these conditions is as great or greater than that between provinces, although there are clearly significant constraints on the inferred polybaric to low-pressure isobaric fractionation regimes. Covariation of total FeO, Ti O-2, La/Yb, Ce/Sm, Zr/Y and Zr/Nb reflects small but significant differences in bulk composition and ambient melt fraction while the covariance of Ce/Srn and Sm/Yb is consistent with the segregation of primitive melts at the spinel-to gamet-lberzolite transition. (c) 2006 Elsevier B.V. All rights reserved.