The Pleistocene adakitic rocks in the western Panamanian arc: slab melt or evolved mantle-derived magma?

Abstract

The Pleistocene volcanic rocks from western Panama and southern Costa Rica, Central America are characterized by adakite-like geochemical features of high Sr/Y (>50) and La/Yb (≥20) with low Y (<15 ppm), low Yb (<1.5 ppm) and low 87Sr/86Sr (<0.704). Early studies argue that they are slab melts with various degrees of mantle interaction during ascent whereas recent studies suggest that they are formed by high-pressure (≥30 km depth) differentiation of mantle-derived basaltic arc magma.

In order to better understand their petrogenesis and test the previously suggested hypotheses, we have investigated major and trace element and Nd, Sr and Pb isotope geochemistry and mineralogy of seven volcanic rocks of western Panama where the Cocos ridge subducts.

Our results show that they are characterized by enrichments in LILE with high LREE abundances and pronounced depletions in the HREE and HFSE, confirming the large influence of enriched slab components suggested by previous studies. The Nd, Sr and Pb isotope data are consistent with those of arc basalts that erupted at about 2 Ma prior to the western Panamanian adakitic rocks in the same region, which originated from enriched mantle metasomatized by melts from the subducting Cocos ridge. However, they have slightly more radiogenic Sr isotopic ratios than the Cocos Ridge basalts and other Galapagos OIBs.

The compositions of amphibole phenocrysts from the adakitic rocks yield pressure, temperature and fO2 conditions of ~ 220-960 MPa, ~ 721-1009 °C, ~ NNO - 0.1-NNO + 2.2, suggesting the magmas have experienced crystallization at multiple depths with large variations in temperature and fO2 conditions. Crystallization pressure and temperature of clinopyroxene and orthopyroxene phenocrysts from basaltic rocks of 600-1300 MPa and 1020-1170 °C respectively show early-stage magma differentiation in lower crust or at crust-mantle boundary beneath the southern Central American volcanic arc (CAVA).

The calculated melts in equilibrium with high pressure amphiboles (≥920 MPa) have a wide range of Mg# (0.31-0.56) and SiO2 (58.4-71.3 wt.%), which reflects significant degree of magma differentiation within a crustal magma storage region (crystal mush zones) underneath the western Panamanian arc. The Mg# of melts in equilibrium with clinopyroxene phenocrysts in the mafic and felsic rocks range from 0.41 to 0.68 and they are consistent with Mg# (0.51-0.66) of the basalt and basaltic andesites in this study, suggesting that pyroxene phenocrysts in the dacite are from a cognate basaltic magma. Petrology, mineral chemistry and whole-rock data of the western Panamanian adakitic rocks indicate the extensive magma evolution from basalt to dacite at high pressure (~35-40 km depth).

We performed trace element and isotope modeling to examine the previously suggested models for the formation of the adakitic rocks from the southern CAVA. The results show that the compositional variations of the Pleistocene adakitic rocks from western Panama and southern Costa Rica are best explained by garnet- and amphibole- dominated fractional crystallization of basaltic arc melts whose mantle source was metasomatized by slab melts from the subducting Cocos ridge. This is consistent with the adakitic features of the calculated melts in equilibrium with high pressure amphiboles and diagnostic trace elements (La/Yb, Yb, Nb/Yb, Sm/Yb, Dy/Yb) that show co-crystallization of garnet and amphibole in the adakitic rocks from western Panama and southern Costa Rica during magma evolution.

The presence of amphibole-rich cumulative enclave, disequilibrium textures and compositions in the phenocrysts of the evolved adakitic rocks are consistent with magma mixing and low pressure fractional crystallization models, suggesting that they may also have influenced on their geochemical variations of the Pleistocene adakitic rocks from western Panama and southern Costa Rica.