Fractional crystallization (geology)

Geology

Hornblende

Biotite

Continental arc

Petrology

Volcano

Basalt

Continental crust

Geochemistry

Paleontology

Ultramafic rock

Mafic

Volcanic rock

Felsic

Crust

Zircon

Quartz

Igneous rock

Gabbro

Journal of Petrology

Igneous complexes with a wide variety of lithologies in continental magmatic arcs provide a critical insight into the composition and formation mechanism of continental crust. We report, for the first time, a systematic dataset for the petrology, mineralogy, geochronology, and geochemistry of the Cuijiu Igneous Complex in the eastern Gangdese Arc of southern Tibet. This complex includes a wide variety of rocks from ultramafic–mafic and felsic cumulates to mafic to felsic non-cumulate plutonic rocks. Cumulate and non-cumulate rocks can be distinguished by combining petrography and bulk-rock composition. Titanite and zircon U–Pb dating of 15 samples shows that the varying lithologies within this complex crystallized contemporaneously at c. 200 Ma. The non-cumulate plutonic rocks were formed through hornblende-dominated fractional crystallization from cogenetic primitive basaltic magmas, which were derived from partial melting of a mantle wedge metasomatized by subduction-related fluids. The ultramafic–mafic and felsic cumulates have similar isotopic compositions and crystallization order of minerals to the non-cumulate plutonic rocks, representing complementary compositions of the fractional crystallization processes. Petrological and geochemical signatures call upon two-step fractional crystallization as the dominant process to generate the Gangdese Arc during the Early Mesozoic. The assemblage and crystallization order of minerals, as well as Al-in-Hb geobarometry, reveal that the c. 200 Ma Cuijiu Igneous Complex crystallized at a range of pressures between 10 and 2·5 kbar. This result allows us to propose that the Early Mesozoic Gangdese crust had a normal thickness (∼35 km), in which the middle to lower part (20–35 km) was dominated by hornblende-rich rocks with minor diorites and tonalites intruding metamorphosed Late Paleozoic igneous rocks, followed upward by a thick granitoid batholith (4–20 km) and by volcano-sedimentary rocks (0–4 km).

Constructing the Early Mesozoic Gangdese Crust in Southern Tibet by Hornblende-dominated Magmatic Differentiation