Rb-Sr ages of granitic rocks in southeastern China and their tectonic significance

Rb-Sr age data and initial Sr⁸⁷/Sr⁸⁶ ratios (I values) are reported for the Mesozoic granitic batholiths along the coast of southeastern China. Two major thermal episodes are recognized: 90 to 120 m.y. B.P., derived from the mineral isochrons, and 165 ± 13 (2σ) m.y. B.P., derived from a whole-rock isochron. These episodes appear to be correlative with periods of rapid spreading of the Mesozoic Pacific Ocean floor and embrace two orogenic phases of the Yenshan orogeny.

The I values of granitic gneisses (0.7060 to 0.7159) vary systematically with the Rb/Sr ratios of their whole-rock samples. The high I value (0.7112) of a pegmatite dike cutting a granitic gneiss in Chinmen suggests that the pegmatite is derived from remelting of upper crustal rocks. The narrow range of I values for more mafic intrusive bodies in Matsu (0.7065 to 0.70695) suggests that they are genetically related and are probably derived from an upper mantle source and have some crustal contamination. An un-metamorphosed granitic sample from Changlo has a mineral isochron age of about 120 m.y., which is interpreted as the time of intrusion. Its rather low I value (0.70546) suggests that the magma source is likely in the upper mantle.

The Yenshan orogenic belt in southeastern China may belong to the inner belt of the Pacific-type orogeny; it is characterized by polycyclic deformation, magmatism, and regional metamorphism. Unlike a classical geosynclinal regime, the magmatic activities in this region were not immediately preceded by a major marine geosynclinal subsidence. The apparent absence of marine deposits after Triassic time suggests that marine transgression is not a universal result of rapid sea-floor spreading. The extensive development of magmatic belts along the eastern margin of the Asian continent appears to require large-scale consumption of oceanic lithosphere at the edge of the continental plate. The age distribution and structural trend data of the magmatic rocks suggest that the presumed subduction zone may have dipped westward or northwestward in the western Pacific region during late Mesozoic time.