Petrogenesis and geodynamic implications of the early Paleozoic potassic and ultrapotassic rocks in the South China Block

Abstract

In this paper, some potassic and ultrapotassic rocks in the South China Block (SCB) have been recognized, according to a set of new geochronological, geochemical and Sr-Nd isotopic data. Zircon U-Pb dating from six plutons yield consistent crystallization ages of 445–424Ma. These potassic and ultrapotassic rocks can be geochemically subdivided into three groups. Group 1, represented by the Longchuan gabbro, longmu diabase, Tangshang and Danqian diorite (445–433Ma), have low silica contents (SiO2=47.38–54.16wt.%), and high MgO (4.21–9.51wt.%) and total alkalis (Na2O+K2O=3.08–5.57wt.%), with K2O/Na2O ratios of 0.62–1.82. They are enriched in LREE and depleted in Ba, Sr and Ta-Nb-Ti, and exhibit relatively high initial 87Sr/86Sr ratios (0.70561–0.71128), low εNd(430Ma) values (−8.4 to −3.2), suggesting that they were most plausibly generated by the partial metling of enriched mantle source (EMI). Group 2, from the Huwei diorite (424Ma), have 45.68–52.87wt.% of SiO2, 5.79–9.25wt.% of MgO and 52–65 of mg-number. They have significantly higher Th (9.92ppm), Ce (88.0–115ppm) concentration and Ce/Yb (27.6–46.8), Th/Yb ratios (2.58–7.99), and relatively low initial 87Sr/86Sr ratios (0.70501–0.70599), and high εNd(430Ma) values (−2.1 to −1.5). We propose that they originated from the partial melting of the depleted mantle source with subsequent contamination by crustal materials. Group 3, represented by the Daning lamprophyre (∼445Ma), has SiO2 contents ranging from 41.73wt.% to 45.22wt.%, MgO from 13.74wt.% to 15.16wt.%, and mg-muber from 73 to 77, with high K2O/Na2O ratios (>2.0). They have 87Sr/86Sr ratios of 0.62912–0.70384 and εNd(t=430Ma) values of −6.4 to −6.3, indicating that the source components are close to the EMI source, with significant sediments involved. These Silurian potassic and ultrapotassic rocks in the SCB can be responsible for post-orogenic delamination and intra-plate extension. And the delamination had a small size and a long duration, and a negligible impact.