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Basalt and tonalite from the Amami Plateau, northern West Philippine Basin: New Early Cretaceous ages and geochemical results, and their petrologic and tectonic implications
Author(s) -
HickeyVargas Rosemary
Publication year - 2005
Publication title -
island arc
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.554
H-Index - 58
eISSN - 1440-1738
pISSN - 1038-4871
DOI - 10.1111/j.1440-1738.2005.00474.x
Subject(s) - geology , terrane , magmatism , basalt , subduction , geochemistry , plateau (mathematics) , island arc , cretaceous , isochron , hornblende , tectonics , paleontology , biotite , mathematical analysis , quartz , mathematics
Basalts and tonalites dredged from the Amami Plateau in the northern West Philippine Basin have the geochemical characteristics of intraoceanic island arc rocks: low 87 Sr/ 86 Sr (0.70297–0.70310), intermediate 143 Nd/ 144 Nd (0.51288–0.51292), moderate light rare earth element (LREE) enrichment (La/Yb = 4.1–6.6) and high La/Nb (1.4–4.3). The incremental heating of hornblende from tonalites yielded well‐defined plateaus and 40 Ar/ 39 Ar isochron ages of 115.8 ± 0.5 Ma and 117.0 ± 1.1 Ma, while plagioclase yielded disturbed Ar release patterns, with ages ranging from 70 to 112 Ma. Taken together, these results show that the Amami Plateau was formed by subduction‐related magmatism in the Early Cretaceous period, earlier than indicated by prior K/Ar results. The results support tectonic models in which the West Philippine Basin was opened within a complex of Jurassic–Paleocene island arc terranes, which are now scattered in the northern West Philippine Basin, the Philippine Islands and Halmahera. The Amami Plateau tonalites and basalts have higher Sr/Y and lower Y and 87 Sr/ 86 Sr compared with younger tonalitic rocks from the northern Kyushu–Palau Ridge and the Tanzawa complex, which were formed by the subduction of the Pacific Plate beneath the Philippine Sea Plate. Based on the geochemical characteristics of the basalts, the Early Cretaceous subduction zone that formed the Amami Plateau may have been the site of slab melting, which suggests that a younger and hotter plate was being subducted at that time. However, the Amami tonalites were probably formed from basaltic magma by fractional crystallization or by partial melting of basaltic arc crust, rather than by melting of the subducted slab.