RECENT GEOCHEMICAL CONSTRAINTS ON THE WESTERN PANAMA CANAL BASIN VOLCANIC ARC ROCKS

Panama arc activity has been divided into three episodes, the depleted Late Cretaceous to Eocene episode, the enriched Miocene episode, and the Pliocene to recent adakite suite. Rocks from the oldest episode are dominantly hornblende bearing, contain a large negative Nb-Ta anomaly, and exhibit enrichment of large-ion lithophile elements (LILEs). These characteristics are all indicators that the Late Cretaceous-Eocene episode rocks were derived from a hydrous mantle wedge subduction zone magma. Miocene episode rocks are commonly calc-alkaline throughout Panama. In contrast, Miocene episode rocks from the Panama Canal (PC) are strongly tholeiitic, lack hydrous minerals, have a decreased Nb-Ta subduction zone signal, and exhibit LILE depletion relative to older PC volcanism as well as the older Panama arc episodes.

Recent chemistry performed at Florida State’s National High Magnetic Field Laboratory has produced new trace element and isotope data from the Western Canal Basin (WCB) between the El Valle volcano and the PC. The heavy rare earth element signatures are greater than or equal to PC samples and are among the highest in all of Panama. The Ba/Yb values for WCB samples are between the depleted PC samples and fluid rich El Valle samples. The Ta/Yb values are similar to PC samples, with both exhibiting a lesser subduction zone signal than the Cretaceous-Eocene arc. On a V versus Ti tectonic discrimination diagram both PC and WCB samples plot in the MORB/BAB/CFB field. Across the Panama Canal Basin (PCB) there are strong geographic correlations with isotopic data and certain trace element ratios. From east to west Pb²⁰⁷/Pb²⁰⁴ decreases from 15.57 to 15.54, Pb²⁰⁶/Pb²⁰⁴ from 19.01 to 18.86 and Pb²⁰⁸/Pb²⁰⁴ from 38.7 to 38.45. In contrast, εHf increases from 10.65 to 13.15 east to west. Both the Pb and Hf isotopic data indicate a westward decrease in the mantle OIB component and an increase in the MORB-like component across the PCB.

The WCB and PC volcanics are interpreted to have formed due to shallow melt generation via decompression melting caused by extensional forces induced by fracturing and rotation of the Panama block post collision with South America. In addition, gravity data collected in 2012 has been used to model the WCB as a series of horst and grabens buried under volcanics and sedimentary deposits.