Petrogenesis Of Intrusions Research Articles

Geochronology and Petrogenesis of Mafic-Intermediate Intrusions on the Northern Margin of the Central Tianshan (NW China): Implications for Tectonic Evolution

Late Paleozoic mafic-intermediate intrusions are widespread in the central Tianshan belt, but their tectonic settings remain controversial. Petrogenesis of these intrusions can provide insights into the tectonic evolution of the belt and its adjacent areas. This study presents new whole-rock geochemical and zircon U-Pb geochronology and Hf isotopic data for the Yaxi diorite and Qianzishan gabbro on the northern margin of the central Tianshan (NW China). Zircon U-Pb dating yielded the magma crystallization ages of 313±4 and 295±4 Ma for the Yaxi diorite and Qianzishan gabbro, respectively. They have lower Mg# values, Ni and Cr contents than typical mantle-derived primary melt, with negative correlations between MgO, TFeO and SiO2 contents, indicating clinopyroxene and olivine fractionation during magma evolution. They are characterized by enrichment of large ion lithophile elements (e.g., Rb, Ba and Sr) and depletion of high field strength elements (e.g., Nb, Ta and Ti) with high Ba/Th and Rb/Y, suggesting that their mantle sources had been metasomatized by slab-derived fluids. In addition, the Qianzishan gabbro has high Al2O3 contents (19.54 wt.%-20.88 wt.%) and positive Eu anomalies (Eu/Eu*=1.09-1.42), which can be attributed to accumulation of plagioclase. Geochemical and zircon Hf isotopic compositions reveal that both the Yaxi diorite and Qianzishan gabbro were derived from depleted lithospheric mantle in the spinel stability field with insignificant crustal contamination. In association with previous investigations, we suggest that the Yaxi and Qianzishan intrusions were emplaced in a subduction-related environment, which means that the subduction of the Junggar Ocean lasted at least to the earliest Permian.

Geochemistry, petrogenesis, and tectonic setting of the Almogholagh batholith in the Sanandaj–Sirjan zone, western Iran

The Almogholagh batholith in the northern Sanandaj-Sirjan magmatic–metamorphic zone comprises three intrusive bodies (gabbroic diorite, quartz syenite, and quartz monzonite) that were generated during the northeastward subduction of Neo-Tethys beneath the Iranian sector of the Eurasian plate. These bodies intruded at different time phases and are related to post-collision magmatism. The quartz syenite and quartz monzonite rocks with specifications of metaluminous, generally ferroan, alkalic to alkali-calcic types, high content of Na2O + K2O, Zr, Ce, Ga, Y, Nb, Ta, and rare earth elements, and depleted in Eu, Sr, and Ti show borderline characteristics between A1 and A2 types granitoids but with more affinity to A2 type. The gabbroic dioritic rocks show borderline specifications between A1 and I types rocks but with more affinity to I type. Distinctive spiked peak patterns in spider diagrams accompanied by (La/Yb)CN values equal to 2.44 to 6.11 and a Ba/La ratio >3 indicate the magmatism activity in the volcanic arc environment. The characteristics (Ba/Rb)PN < 1, (Ba/Th)PN < 1, and Th/Ta ratio from 3.18 to 8.42 suggest the magmatism activity of the continental margin setting. The specifications of post-collision magmatic activities, 143Nd/144Nd > 0.512638 in some samples, εtNd > 0, εtSr > 0, and high content of Nb, Ta, and Zr (589 ppm) demonstrate the involvement of the mantle source, subducted slab fluids, high flux of mantle-derived halogen-rich volatiles, and contamination within the crust during the petrogenesis of intrusions. After the initial collision, the operation of minor subduction (with slab break-off) or foundering of the lithospheric mantle (delamination) occurred because of asthenospheric upwelling and heat flows in the mantle in the Sanandaj-Sirjan zone. Stretch and local disruptions were created by these heat flows; simultaneously, magma was formed and ascended upward.

Geochemistry and petrogenesis of intrusions at the Golden Pride gold deposit in the Nzega greenstone belt, Tanzania

The greenstone sequence at Golden Pride gold deposit in Nzega greenstone belt of northern Tanzania is cross-cut by several intrusions whose geochemistry and petrogenesis is unknown. We present major and trace elements geochemistry of the Golden Pride igneous intrusions with the aim of constraining their petrogenesis and their ancient tectonic setting. The Golden Pride intrusions are geochemically categorized into two main rock suites: the granodiorites (which include the porphyries) and the lamprophyres (formerly intermediate intrusions). The granodiorites are characterized by SiO2 contents of 54.5–69.9wt%, elevated MgO (1.22–3.59wt%) Cr (up to 54ppm), Mg# (35–55) pointing to a mantle component in the source. Compared to the TTG and adakites, the granodiorites are characterized by higher K2O (1.52–4.30wt%), medium HREE (Gd/ErCN=2.13–3.77) and marked enrichment in Ba and Sr (Ba+Sr=819–2922ppm) and are in these respects similar to Archean high Ba–Sr sanukitoids. The rocks in this suite are interpreted to have formed by partial melting of an enriched mantle wedge through two metasomatic events: subduction-related fluids/melts and by metasomatism related to asthenospheric mantle upwelling caused by slab break-off.Compared to the granodiorites, the lamprophyres have higher MgO contents (2.37–3.81wt%), Cr (60–298ppm), Co (31–57ppm) and Mg# (32–40). They also show slight enrichment of the LREE relative to HREE (La/YbCN=3.3–7.1), moderate Nb–Ti depletion and sub-chondritic Zr/Hf ratios (34–41). These geochemical features are attributed to derivation of the Golden Pride lamprophyres by partial melting of the amphibole-rich metasomatized mantle by slab derived hydrous fluids. Both of the Golden Pride intrusion suites show strong affinity to subduction related magmas and we interpret that the entire greenstone sequence and the associated intrusions at Golden Pride gold deposit formed in a late Archaean convergent margin.