Petrochemistry and petrology of I-type granitoids in an arc setting: the composite Torul pluton, Eastern Pontides, NE Turkey

Kaygusuz A., Siebel W., Sen C., Satir M.

INTERNATIONAL JOURNAL OF EARTH SCIENCES, vol.97, no.4, pp.739-764, 2008 (SCI-Expanded) identifier identifier


The Upper Cretaceous Torul pluton, located in the Eastern Pontides, is of sub-alkaline affinity and displays features typical of volcanic arc granitoids. It is a composite pluton consisting of granodiorite, biotite hornblende monzogranite, quartz monzodiorite, quartz monzonite and hornblende biotite monzogranite. The oldest syenogranite (77.9 +/- 0.3 Ma) and the youngest quartz diorite form small stocks within the pluton. Samples from the granodiorites, biotite hornblende monzogranites, quartz monzodiorites, quartz monzonites and hornblende biotite monzogranites have SiO2 between 57 and 68 wt% and display high-K calc-alkaline, metaluminous to peraluminous characteristics. Chondrite-normalized REE patterns are fractionated (La-cn/Lu-cn = 6.0-14.2) with pronounced negative Eu anomalies (Eu/Eu* = 0.59-0.84). Initial epsilon(Nd(i)) values vary between -3.1 and -4.1, initial Sr-87/Sr-86 values between 0.7058 and 0.7072, and delta O-18 values between +4.4 and +7.3 degrees. The quartz diorites are characterized by relatively high Mg-number of 36-38, low contents of Na2O (2.3-2.5 wt%) and SiO2 (52-55 wt%) and medium-K calc-alkaline, metaluminous composition. Chondrite-normalized REE patterns are relatively flat [(La/Yb)(cn) = 2.8-3.3; (Tb/Yb)(cn) = 1.2] and show small negative Eu anomalies (Eu/Eu* = 0.74-0.76). Compared to the other rock types, radiogenic isotope signatures of the quartz diorites show higher Sr-87/Sr-86 (0.7075-0.7079) and lower epsilon(Nd(i)) (-4.5 to -5.3). The syenogranites have high SiO2 (70-74 wt%) and display high-K calc-alkaline, peraluminous characteristics. Their REE patterns are characterized by higher La-cn/Lu-cn (12.9) and Eu/Eu* (0.76-0.77) values compared to the quartz diorites. Isotopic signatures of these rocks [epsilon(Nd(i)) = -4.0 to -3.3; Sr-87/Sr-86((i)) = 0.7034-0.7060; delta O-18 = + 4.9 to + 8.2] are largely similar to the other rock types but differ from that of the quartz diorites. Fractionation of plagioclase, hornblende, pyroxene and Fe-Ti oxides played an important role in the evolution of Torul granitoids. The crystallization temperatures of the melts ranged from 800 to 900 degrees C as determined from zircon and apatite saturation thermometry. All these characteristics, combined with low K2O/Na2O, low Al2O3/(FeOT + MgO + TiO2), and low (Na2O + K2O)/(FeOT + MgO + TiO2) ratios suggest an origin through dehydration melting of mafic lower crustal source rocks.