|Figure 10: Map of rock ages in southeastern China from Sm-Nd model ages. Several different provinces of different geochemical history make up southeastern China and are represented by the roman numerals. Province VI contains both coastal Fujian and Taiwan. Both are part of the larger province of Cathaysia (including III, IV, V, and VI). After Chen and Jahn (1998).|
Southeastern China can be divided into six geochemical provinces (figure 10). Taiwan is part of the Cathaysia province which includes the extreme southeastern portion of China. Specifically, Taiwan belongs to the same geochemical province as the Fujian province. They share the same early geochemical history and the rocks of the Tananao complex in Taiwan are isotopically similar to rocks in Fujian (Chen and Jahn, 1998).
Figure 11: Nd isotopic evolution for Taiwan metapelites and granitoids of the Tananao metamorphic complex, intraplate basalts of northwestern Taiwan and intraplate basalts of northern Taiwan. Roman numerals explained in the text. After Lan et al. (1996).
The mantle model ages of all rocks in Taiwan are Proterozoic (figure 11): 0.9-1.5 Ga for granitoids and 1.1-2.6 Ga for metapelites (Lan et al., 1996). For this time period, Taiwan and southeastern China show the same isotopic histories. This shared history lasted through the Paleozoic during which southeastern China is interpreted as epeiric platform. Sr and paleontologic data for 242 Ma marbles below the coastal plain of Taiwan correlate to upper Permian rocks of southeastern China. Melting events of Chinese continental crustal rock formed Taiwan's Paleozoic and Mesozoic granitoids (Chen and Jahn, 1998). This melting event is interpreted from preserved Pb-Pb ages; it was probably due to a thermal pulse related to the accretion of Cathaysia to its neighboring Yangtze province (Lan et al., 1996).
By the late Mesozoic, Taiwan started on a different geochemical path than southeastern China (figure 11, I). 175 Ma metapelites show younger Sm-Nd ages than equivalent Chinese rocks, and this is interpreted as evidence of mantle input either due to accretion or a basin closing (Chen and Jahn, 1998; Lan et al., 1996). 77 Ma granites and amphibolites show even younger Sm-Nd ages and higher epsilon neodymium. These rocks' association with ophiolites that a subduction zone existed underneath Taiwan at this time (figure 11; II).
Paleogene metabasites show a transition from this subduction regime to the extensional regime that created the South China Sea. This is also reflected by basalt and andesite in Eocene and Miocene shales of the Central Ranges recording an epsilon neodymium measurment of + 1.3 (figure 11; III). Intraplate basalts under the Western Foothills of northwestern Taiwan show an epsilon neodymium of + 4.9-5.5 (figure 11; IV). This very positive value indicates a major asthenospheric mantle source for these basalts. These basalts (0.1-2.8 Ma) also exhibit high HFSE/LREE (High Field Strength Elements/Light Rare Earth Elements). This is a characteristic of OIB (Ocean Island Basalt) chemistry rather than arc magmatism. The OIB-like chemistry is likely related to extension in this area (Lan et al., 1996), which fits with the location of these basalts in the Okinawa Trough.
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