THE DISCOVERY AND GEOLOGICAL SIGNIFICANCE OF MIDDLE TRIASSIC SHAMA GABBRO IN THE ZHONGZA BLOCK, CHINA

Abstract

The subduction polarity, precise collisional timing, and Late Paleozoic–Early Mesozoic tectonic evolution of the eastern Paleo‑Tethyan Jinshajiang Ocean remain highly controversial, restricting our comprehensive understanding of the tectonic framework and crust‑mantle interaction of the Zhongza Block. To resolve these key scientific issues, this study systematically investigates a newly discovered Middle Triassic gabbro pluton in the Shama area of Batang County, located in the central Zhongza Block, eastern Paleo‑Tethyan tectonic domain, through whole‑rock major and trace element geochemistry, Sr–Nd isotopic analysis, and zircon U–Pb–Hf isotope geochronology. Petrographic observations indicate that the Shama gabbro is mainly composed of plagioclase, diopside, amphibole, and minor K‑feldspar. Geochemically, the samples are characterized by low SiO₂ (45.2 wt.%–45.9 wt.%) and high MgO (8.24 wt.%–9.33 wt.%) contents, belonging to the tholeiitic series with insignificant Eu and Ce anomalies. These gabbros are enriched in large‑ion lithophile elements (Rb, Th, U, Pb) and exhibit no obvious depletion of high‑field‑strength elements (Ta, Nb, Ti). The whole‑rock initial ⁸⁷Sr/⁸⁶Sr ratios range from 0.708700 to 0.709559, with homogeneous negative ε_Nd(t) values (−4.30 to −4.24) and two‑stage Nd model ages (t_DM2(Nd)) of 1.36 Ga. Zircon ε_Hf(t) values vary from −0.46 to 2.81, yielding two‑stage Hf model ages (t_DM2(Hf)) of 945–1124 Ma. These isotopic features demonstrate that the parental magma originated from partial melting of depleted lithospheric mantle and experienced variable contamination by Mesoproterozoic ancient crustal materials. Zircon U–Pb dating defines a robust Middle Triassic formation age of 245 ± 1 Ma for the Shama gabbro. Integrated geochemical and regional geological evidence constrains a post‑orogenic extensional tectonic setting for the pluton, indicating that the arc‑continent collision between the Jinshajiang oceanic plate and the western Zhongza Block was completed before the Early–Middle Triassic, rather than occurring synchronously. The widespread Early–Middle Triassic intracontinental extension of the Zhongza Block is genetically linked to the back‑arc spreading of the southern Cimmerian Continent and the initial opening of the Neo‑Tethyan Ocean during the Permian, which corresponds to the global mantle dynamic evolution associated with Pangea breakup. This study provides new precise geochronological and geochemical constraints on the subduction‑collision process of the Jinshajiang Ocean and the Mesozoic tectonic evolution of the eastern Paleo‑Tethys, and further clarifies the regional geodynamic response to Neo‑Tethys initiation.