| Costa Rican magmatism is somewhat unusual because of the occurrence of high-silica volcanic deposits in an area where a continental crust is absent. However, the ash-flow deposit of the Valle Central, Costa Rica consists in part of high-silica deposits. The Valle Central Tuff covers about 785 km 2 and has a volume of about 22 km3. Based on analyses of pumice fragments, the tuff can be divided into three chemical groupings: a low-silica group (54--62 wt. % SiO2), a high-silica group (63--69 wt. % SiO2), and a mingled group (56--65 wt. % SiO2). Different Eu/Eu*, MREE, and HREE trends in the high and low-silica groups show that these groups can not be related to each other through fractional crystallization. The low-silica magma represents a mantle melt that has undergone fractional crystallization, producing continuous variation in silica, ranging from 53 wt % to 63 wt % SiO2. The high-silica magmas represent melting of amphibolite at the base of a thickened oceanic crust. However, based on uniform potassium enrichments and low variation of REE patterns, the high-silica magma and the low-silica magma are related, and have a common source. We suggest that the high-silica melts resulted from partial melting of meta calc-alkaline rocks, which were similar to the low-silica group within El Valle Central Tuff. The mingled group represents the physical and chemical mixing between these two end-members. |
