期刊
INTERNATIONAL GEOLOGY REVIEW
卷 43, 期 2, 页码 120-138出版社
TAYLOR & FRANCIS INC
DOI: 10.1080/00206810109465004
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Rare-earth-element. radiogenic and oxygen isotope, and mineral chemical data are presented for tholeiitic and alkaline Quaternary volcanism from Karasu Valley (Hatay. southeastern Turkey). Karasu Valley is the northern segment of the Dead Sea transform fault and is filled with flood-basalt type volcanics of Quaternary age. This valley is an active fault zone that is known as Karasu fault, extending in a NE-SW direction. The Karasu Valley basaltic volcanics (KVBV) are sub-aphyric to porphyritic. with variable amounts of olivine, clinopyroxene. and plagioclase phenocrysts. Alkali basalts are generally characterized by high contents of olivine, clinopyroxene. and plagioclase phenocrysts. Their groundmass contains olivine, clinopyroxene, plagioclase, and Fe-Ti oxides. Tholeiitic basalts are sub-aphyric to porphyritic (high contents of olivine. clinopyroxene. and plagioclase). Their groundmass is similar to that tb alkali basalts. The range of olivine phenocryst and microlite compositions for all analyzed samples is Fo(81) to Fo(43). Plagioclase compositions in both tholeiitic and alkali basalts range from andesine, An(38) to bytownite, An(72). Clinopyroxene compositions range from diopside to calcic augite. Most of the olivine. plagioclase, and clinopyroxene phenocrysts are normally zoned and/or unzoned. Fe-Ti oxides in both series are titanomagnetite and ilmenite. Based on normative and geochemical data, the Karasu Valley basaltic volcanics are mostly olivine and quartz-tholeiites. and relatively lesser amount of alkali olivine-basalts. KVBV have low K2O/Na2O ratios, typically between 0.25 and 0.45. Olivine- and quartz-tholeiites are older than alkali olivine-basalts. Olivine tholeiites have Zr/Nb and Y/Nb ratios similar to alkaline rocks, but their Ba/Nb. Ba/La. and La/Nb ratios are slightly higher than alkali olivine-basalts. In contrast, quartz-tholeiites have the highest Ba/Nb, Ba/La, Zr/Nb, and Y/Nb and the lowest Nb/La ratios among the KVBV. Alkali basalts have Sr-87/Sr-86 and Nd-143/Nd-144 ratios ranging from 0.703353 to 0.704410 and 0.512860 to 0.512910. respectively. In contrast, quartz-tholeiites have higher Sr-87/Sr-86 and lower Nd-143/Nd-144 ratios. which vary from 0.704410 to 0.705490 and 0.512628 to 0.512640, respectively. Olivine tholeiites have intermediate isotopic compositions ranging from 0.703490 to 0.704780 anti 0.512699 to 0.512780, respectively. Pb-206/Pb-204, Pb-207/Pb-204 and Pb-208/Pb-204 isotopic ratios of KVBV range from 18.817 to 19.325, 15.640 to 15.718. and 39.054 to 39.223, respectively. The range of O isotope values is between +5.84 and +7.97 parts per thousand. The higher O and Sr isotopes in olivine- and quartz-tholeiites relative to alkali olivine-basalts can be explained by contamination of magmas by crustal materials. The KVBV have intraplate chemistry similar to that of other tholeiitic: and alkaline basalts in other within-plate environments. and isotopes range from isotopically depleted mantle to enriched isotope compositions similar to some enriched ocean islands. Trace-element and isotope data indicate that the KVBV are derived from a common OIB-tike asthenospheric mantle source, but they have experienced different degrees of crustal contamination during their ascent to the surface, contemporaneous with little fractional crystallization. although quartz-tholeiites display significant effects of crustal contamination, alkali olivine-basalts appear to have negligible or no crustal contamination in their genesis.
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