Crustal thickening and uplift of the Qiangtang Terrane, Tibetan Plateau during the Late Cretaceous to Early Palaeocene: geochronology and geochemistry of the Saiduopugangri granite

There continues to be debate regarding the timing of the collision between the Indian and Eurasian plates and the uplift of the Tibetan Plateau. This study presents zircon U–Pb geochronology, whole-rock geochemistry, and Lu–Hf isotopic data for the Saiduopugangri granite of the Qiangtang Terrane, located within the core of the Tibetan Plateau. These data provide the basis for the geodynamic setting, petrogenesis, and characteristics of its magma source. Zircons from the Saiduopugangri granite yield a weighted-mean 206Pb/238U age of 62.72 ± 0.06 Ma, indicating that these rocks formed during the early Palaeocene. The rocks are members of the highly calc-alkaline to shoshonitic series, with weak peraluminous characteristics. Trace elements are characterised by high Sr (483–616ppm), and low Y (6–10ppm) and Yb (1ppm) content, typical of a high Sr and low Yb granite. The εHf(t) of zircon range from −2.14 to 2.35, with two-stage Hf model ages (TDM2) ranging from 1182 to 895Ma. These data suggest that the Saiduopugangri granite magma was derived from the melting of lower-crustal clastic meta-sedimentary rocks and mantle-derived basalts. The high Sr and low Yb granite characteristics and experimental results indicate that melting occurred at >1.2 GPa and >750 °C, consistent with a crustal thickness greater than 50km. Magmatism occurred from the Late Cretaceous to the early Palaeogene and is broadly synchronous with the collision timing between the Indian and Eurasian plates. The Saiduopugangri granite provides evidence of crustal thickening of the Tibetan Plateau and its age and petrogenesis constrain the timing of the initial uplift.

Petrology and Geochemical Characteristic of Granitoids From Guéra Massif in the Central Part of Chad: An Example of Mixing Magmas

The granitoids of Guéra Massif are composed of biotite-granite, amphibole-biotite granite and gabbro-diorite and commonly contain micro granular mafic enclaves which vary from monzogabbro to syenite composition. They are metaluminous, high-K calc-alkaline to shoshonitic series. Gabbro-diorite rocks are magnesian while amphibole-biotite granites are magnesian to ferroan, and biotite granites are ferroan. They are enriched in LREEs relative to HREE and display negative anomalies in Nb, Ta and Ti. Fields relationships, petrology and geochemistry indicate that mixing and mingling processes could be more relevant for the genesis of granitoids associated to fractional crystallization. Thus, the presence of mafic enclaves of gabbro-diorite composition in the granites, the resumption of alkaline feldspar xenocrystals in the gabbro-diorites, as well as the linear correlation between the granites and the gabbro-diorites and the intermediate position of the mafic enclaves between the two formations, enable us to propose magmatic mixing as the major process that presided over the evolution of the Guéra granitoids. The delamination of the continental lithosphere during the post-collisional phase of the Pan-African orogeny would have caused the partial melting of the subduction-modofied mantle and lower continental crust and thus produced the magmas of the Guéra granitoids.

Petrogenesis and Chronology of Intermediate-Acidic Intrusive Rocks in Tongling Area, Anhui Province, China

The intermediate-acidic intrusive rocks in Tongling area of Anhui province can be divided into two magmatic series, namely the shoshonitic series and the high-potassium(K) calc-alkaline series. The shoshonitic series is characterized by an association of pyroxene monzodiorites + monzodiorites + quartz monzonites, and high Rittmann indices. This series has common association with gold deposits. The high-potassium(K) calc-alkaline series is characterized by an association of diorite + quartz monzodiorites + granodiorites. It has lower Rittmann indices and REEs than the shoshonitic series and is associated with copper deposits. Results obtained from the studies show that the former is derived from the differentiation of the alkaline basic magma in mantle and the later is mixture between the differentiated mantle magma and crustal magma. The chronology of intermediate-acidic intrusive rocks is about 140Ma.

Archean Rocks of Shoshonitic Affinities at Bijou Point, Northwestern Ontario

The Archean volcanic and intrusive rocks of Bijou Point, northwest Ontario show several chemical characteristics which suggest affinities to the alkali or shoshonitic series characteristic of late stages of evolution in modern island arcs. The rocks have unusually high total alkalis (7% at 58–65% SiO2), high Al2O3 and high trace element contents (Ba 800–2500 ppm, Rb 50–350 ppm, Sr 600–2000 ppm). The presence of such rocks in the greenstone belts of the Superior Province serves to further emphasize the similarities between these ancient volcanics and their modern counterparts.