International Geology Review

Pezzino, Antonino; Angì, Gerolamo; Fazio, Eugenio; Fiannacca, Patrizia; Lo Giudice, Antonino; Ortolano, Gaetano; Punturo, Rosalda; Cirrincione, Rosolino; De Vuono, Eloisa

doi: 10.2747/0020-6814.50.5.423pmid: N/A

Structural, petrologic, and thermobarometric data presented in this paper contribute to our understanding of the tectono-metamorphic evolution of the lowest tectonic slices of the Aspromonte Massif (southern Calabria, Italy), which crop out in three main tectonic windows. Despite previously being considered different units, they exhibit the following similar features: the same tectonic evolution, analogous blasto-deformation relationships, and absence of Hercynian mineralogical assemblage relics. Similar P-T paths indicate early HP-LT Meso-Alpine metamorphism (400-600° C at 0.95-1.35 GPa), evolving in the Oligocene-Miocene toward a subsequent retrograde shearing event ranging from 480° to 610°C and 0.50 to 0.95 GPa. The latest retrograde evolution is characterized by 350-480°C and 0.32-0.62 GPa. In this new tectonic framework, it is proposed to group the metapelite sequences defining the Madonna di Polsi Unit. Data presented herein suggest that the pre-Alpine geodynamic setting of southern Calabria was a thinned continental margin made up of Hercynian basement and Mesozoic terrigenous-carbonate sedimentary cover. This continental margin evolved during the early Meso-Alpine stage into a subduction zone beneath the European plate, followed by Neo-Alpine syn-convergent exhumation along a deep-seated mylonitic shear zone. These processes are responsible for the Alpine metamorphic overprint on the Hercynian terranes, as well as for Alpine metamorphism of their Mesozoic cover.

Zhu, Di-Cheng; Pan, Gui-Tang; Chung, Sun-Lin; Liao, Zhong-Li; Wang, Li-Quan; Li, Guang-Ming

doi: 10.2747/0020-6814.50.5.442pmid: N/A

We present SHRIMP zircon dating, bulk-rock geochemical, and Sr-Nd-Pb isotopic results for Yeba volcanic rocks and a mafic dike from Southern Gangdese (SG), southern Tibet, in order to constrain their tectonic setting and origin. Yeba volcanic rocks span a continuous compositional range from basalt to dacite, although andesites are minor, and mafic and felsic rocks are volumetrically predominant. New SHRIMP zircon dating for a dacite coupled with previous SHRIMP zircon dating for a mafic dike and fossil constraints for the sedimentary sequence indicate that Yeba volcanic rocks were emplaced in the Early Jurassic (174-190 Ma). Yeba tholeiitic mafic rocks possess compositional diversity and are divided into three groups based on concentrations of MgO, Al2O3, and La. Mafic samples are all characterized by marked negative Nb, Ta, and Ti anomalies and positive εNd(T) values (+ 2.4 to + 4.5). Yeba calc-alkaline felsic rocks are characterized by coherent, concave-upward MREE patterns and negative anomalies in Nb, Ta, P, and Ti, with positive εNd(T) values (+ 0.3 to + 2.6). Sr-Nd-Pb isotopes overlap among the different groups of Yeba mafic rocks; Pb isotopic compositions in both mafic and felsic rocks are nearly identical. These features are consistent with a subduction-related origin, most likely in an arc built on thin, immature continental crust. Yeba volcanic rocks are interpreted as having been created by northward subduction of Neo-Tethyan oceanic crust in Early Jurassic time. Geochemical signatures and quantitative modeling indicate that fractional crystallization and crustal assimilation played insignificant roles in the generation of Yeba mafic magmas, and that their geochemical diversity was probably produced by variable degrees of partial melting from a common but heterogeneous mantle source, which had been metasomatized by variable contributions of sediments/fluids released from the subducted Neo-Tethyan oceanic crust. Yeba felsic rocks were probably generated by moderate degrees of partial melting of juvenile basaltic lower crust, which consists of dominant underplated magmas (similar to Yeba mafic rocks in composition) and variable contributions from ancient lower crust beneath the Gangdese Back-Arc fault uplift belt (GBAFUB).

Zhang, Lianchang; Zhou, Xinhua; Ding, Shijiang

doi: 10.2747/0020-6814.50.5.472pmid: N/A

The Jiaodong super-large gold metallogenic region is located on the eastern margin of the North China craton and the western margin of the Circum-Pacific tectonic belt. Ore-forming chronology indicates that these deposits formed at 120 ± 10 Ma. Helium and argon isotope compositions of fluid inclusions in pyrite range in 3He/4He ratios from 0.38 to 2.36 R/Ra (R: 3He/4He ratios of samples; Ra: 3He/4He ratios of Beijing air, 1.4 x 10-6) and in 40Ar/36Ar ratios from 310 to 1148. δ18O and δD values of fluid inclusions in quartz are -2.16 to +6.28‰ and -65 to -98‰, respectively. A consistent relationship between H-O and He-Ar isotopic systems show that mantle-source fluids were involved in large-scale metallogeny in the Jiaodong region. The fluids were produced accompanying delamination, lithosphere thinning, and crust-mantle interaction that occurred in the eastern North China craton during the Early Cretaceous (140-120 Ma).

Hezarkhani, Ardeshir

doi: 10.2747/0020-6814.50.5.483pmid: N/A

The Sonajil porphyry copper system formed in a small, relatively deep-seated (~5 km) granodioritic intrusion at Ahar, East Azarbaijan, Iran. Based on field geology, petrography and fluid inclusion studies, early hydrothermal alteration caused a potassic-type alteration in the deep, central parts of the intrusion, and generated a peripheral propylitic halo. Dominant phyllic alteration overprinted apical parts of the intrusion. Hypogene copper mineralization was associated with potassic alteration. The earliest fluid circulated in the complex had high salinity (35-60 wt% NaCl equiv.), and is interpreted to have been formed directly from the crystallizing magma. Major entrapment of this fluid, which is responsible for potassic alteration and deposition of chalcopyrite and bornite in thin, discontinuous Group I and II veins, occurred after cooling to about 400°C. The average Cu grade in the potassic alteration zone is between 0.01 and 0.07%. Circulation of a lower temperature fluid having a homogenization temperature of 220-350°C with meteoric H2O that originated in the peripheral parts of the system was responsible for propylitic alteration. During an intermediate phase of hydrothermal activity, meteoric water penetrated central parts of the system along a network of late fractures (Group III veins) and is partly responsible for the Group II veins. Mixing of the later fluid with the magma-derived high-salinity fluid created a fluid characterized by salinities ranging from 1 to 25 wt% NaCl equiv. The inflow of meteoric water, and a continuous temperature decrease within the system, produced more oxidized fluids, which caused phyllic alteration and intense copper leaching. Late boiling in the apical parts of the system favored deposition of sporadic chalcopyrite and bornite, but did not add significant copper. Supergene enrichment led to copper grades higher than that of phyllic zone, but not enough to be of economic significance. Based on the similarities in wall-rock alteration, mineralization style, petrography, and fluid evolution in Sonajil and those of typical productive porphyry copper deposits, the lack of economic mineralization in Sonajil is surprising. It is proposed that this is mainly due to the deep level of emplacement of the intrusion and the dacitic composition of the parental magma. These factors are considered to be the main causes preventing melt saturation with alkali chloride-enriched fluids until the late stage of crystallization, which restricted the amount of exsolved fluid and Cu extracted from the melt. Moreover, overpressuring of the upper parts of the system and a lack of wide apical stockwork veins failed to provide sites for the mineralizing fluids to concentrate copper to economic levels. Finally, the uneconomic nature of the Sonajil porphyry could reflect the lack of multiple intrusions and the intense Cu leaching of the phyllic alteration zone.