Whole-rock Sr-Nd Isotopic Data Research Articles

Genetic pattern of the Albian volcanic rocks in the Ziruco area, Northern Tibet: Implications for A-type granites

Genetic pattern of a continuous volcanic series of andesites, dacites, and rhyolites from the Ziruco area, northern Tibet was debated in this paper to evaluate the later evolution of A-type granites. We present new zircon UPb ages, whole-rock geochemical, zircon Hf isotopic, and whole-rock SrNd isotopic data for these magmatic rocks. Our analysis shows that the zircon UPb ages of the andesites, dacites, and rhyolites are ca. 105.4 Ma, ca.105.3 Ma, and ca.106.8 Ma, respectively. The andesites have high Nb contents (16.3–24.9 ppm) and Nb/U ratios (22.7–32.4), and chondrite normalized REE and primitive-mantle normalized trace element patterns similar to Nb-rich basalts. They were most likely derived from the mantle that had been metasomatized by melts derived from subducted slab and sediment, and then underwent fractional crystallization. The dacites and rhyolites have high Ga/Al ratios (3.05–3.70 and 2.93–3.43, respectively), REE (226–256 and 195–285 ppm, respectively), Nb (30.0–34.4 and 20.3–24.4 ppm, respectively) and Zr contents (443–513 and 147–163 ppm, respectively), and zircon saturation temperatures (856–886 °C and 823–837 °C, respectively), which are similar to A-type granites. The dacites were most likely formed by partial melting of underplated basaltic lower crust with ilmenite and zircon, followed by slight fractional crystallization. The rhyolites were most likely formed by fractional crystallization of the dacite magmas. The minerals of fractional crystallization probably include feldspar, apatite, zircon, and titanite. This continuous volcanic series in Ziruco area may be related to break-off of the Meso-Tethyan slab in northern Tibet during the late Early Cretaceous. Our studies indicate that fractional crystallization is important later evolutionary process for A-type granites from the crustal source area, and it mainly controls the geochemical characteristics of significant negative Ba, Sr, P, Ti, and Eu anomalies of A-type granites from the crustal source area.

Contrasting styles of peraluminous S-type and I-type granitic magmatism: Identification and implications for the accretionary history of the Chinese South Tianshan

Peraluminous granitoids have aluminum saturation indices (A/CNK) higher than 1.0, which overlap to some extent between S- and I-type granitoids. However, their source and petrogenesis are still disputed. For example, whole-rock compositions alone are not always a valid way to discriminate the sources of peraluminous granitoids. To identify the geochemical affinities, source and petrogenesis of the peraluminous granitoids, we present new geochemical data, <i>in situ</i> zircon U-Pb ages and Hf-O isotopic data, and whole-rock Sr-Nd isotopic data for the peraluminous granitoids in the South Tianshan Orogen Belt (STOB), Northwesten China. Zircon U-Pb ages suggest that these peraluminous granitoids were emplaced in the latest Carboniferous (<i>ca.</i> 299 Ma). They contain the diagnostic mineral muscovite and have high δ¹⁸O_Zrn values (&gt;8.0 ‰) demonstrating a close affinity with S-type granitoids. Their low εNd(t) values (−5.3 to −7.6), combined with variable zircon εHf(t) values (−0.35 to −10.18), indicate that these S-type granitoids were likely derived from partial melting of metasedimentary rocks. In addition, inherited zircon cores from the S-type granitoids have variable δ¹⁸O values (6.34–10.5 ‰) and zircon εHf(t) values (−4.3 to +6.3), with age populations (<i>ca</i>. 400 to 500 Ma) similar to those of detrital zircons from late Carboniferous metasedimentary rocks in the region. These data show that the S-type granitoids were dominantly derived from late Carboniferous metasedimentary rocks rather than Precambrian crustal materials. The studied granitoids have a transitional composition between I- and S-type granitoids, which could be related to low compositional maturity of the late Carboniferous metasedimentary source. According to the spatial and temporal distribution and petrogenesis of the Carboniferous intrusive rocks in the STOB, we propose that a slab roll-back model can account for the generation of late Carboniferous S-type granitoids in the STOB.

Geochronology and petrogenesis of the Xiongmei Cu-bearing granodiorite porphyry in North Lhasa subterrane, central Tibet: Implication for the evolution of Bangong-Nujiang metallogenic belt

Bangong-Nujiang metallogenic belt is a recently recognized post-subduction porphyry-skarn Cu polymetallic metallogenic belt in Tibet. The Xiongmei Cu-bearing granodiorite porphyry was newly found in the middle segment of this belt. In this study, we combined zircon U-Pb ages, Hf isotopic, REEs data, whole-rock Sr-Nd isotopic data, and whole-rock geochemical data to constrain the geochronology, petrogenesis and metallogenic features of the Xiongmei Cu-bearing granodiorite porphyry. Zircon U-Pb and whole rock Rb-Sr isochronal age show Xiongmei Cu-bearing granodiorite porphyry formed during 110–105 Ma. The zircon Hf isotope together with whole rock Sr-Nd isotopic features and high Mg# value (39–59), relatively high Cr (10.9–35.5 ppm) and Ni (10–224 ppm), indicating a melt deriving from mixing between mantle material and an ancient crustal source. The possibility of crustal addition in the magma is further supported by varied Eu anomalies (0.5–1.5), Sr anomalies (0.6–1.9) and high Al2O3 contents (15.50–17.25 wt%). Geochemical composition including K2O contents (0.52–4.72 wt%) and K2O/Na2O ratios (0.2–5.5), and the linear relations between LOI, Cu, Rb and Sr contents indicated that the magma sourced from melting of an amphibolite source, and further suggesting that the juvenile crust derived from an enriched lithospheric mantle. The ratios between incompatible elements together with the absence of Ce anomaly suggested the contribution of fluids from subducted oceanic crust rather than melt. The negative Eu and Sr anomaly features together with relatively high HREEs and Y contents, low (La/Yb)N and Sr/Y ratios are interpreted as low pressure partial melting. Combination with the regional geologic background, we adopted a slab break-off model to explains the diverse contemporaneous magmatic rocks recognized in the North Lhasa subterrane including that of the Xiongmei Cu-bearing granodiorite porphyry.