ZIRCON AGES REVEAL UNEXPECTED OLDER FLARE-UP IN THE PERUVIAN COASTAL BATHOLITH

Early researchers suggested that the gabbros of the Rio Pisco area in the Arequipa segment of the Peruvian Coastal Batholith (PCB) were pre-PCB or earliest PCB intrusions and had many similarities with the Patap gabbros in the Lima segment further north. Most of the ages for these gabbros were unknown. These researchers estimated the Arequipa segment resulting from magma flare-up events to be from Albian (105 Ma) through Paleocene (59 Ma). Beckinsale et al. (1985) obtained a Rb-Sr isochron age of 140 Ma from samples collected along the Rio Pisco valley, but rejected it due to the granitoids cross-cutting Albian sediments.

The research reported here investigated our more recently acquired, older-than-expected, U-Pb zircon age of 131 Ma for a gabbro unit in the Cerro Media Luna (CML) area of the Rio Pisco valley. We also investigated the two adjacent granitoid units and the volcaniclastic host rock of the gabbro unit. Field observations, CIPW-norm, and modal analysis show that the granitoid units fit into two distinct groups: the older forms an alkaline trend and the younger a tholeiitic trend. A 133 Ma age for the granitoid near the gabbro and a 127 Ma age for the one further away were obtained. Another age of 132 Ma was obtained for a mafic inclusion from the younger granitoid. We found gradational contacts among the granitoid units and with the gabbro indicating that all are part of the same pluton, corroborating the similar ages. The 127-133 Ma age of the CML unit implies an age older than the previous Albian/Aptian ages assigned to the host rock, the Quilmana Group.

Previously viewed as part of the Linga Intrusive Complex, we propose here that the ca. 25 Ma age difference of the CML units from the other Linga plutons sets the CML units apart from the Linga Intrusive Complex. The 131 Ma gabbro is part of a pre-Linga early intrusive event that includes mafic and intermediate compositions. The cogenetic intrusion of the CML gabbro with granitoids sets the gabbro apart from the Patap Super-unit of the Lima segment. It confirms our earlier interpretation that the mafic units in the Rio Pisco area are gabbros and cumulate bodies of the granitoid super-units and not separate, earlier gabbro units intruded and dissected by them. So, we suggest that the PCB emplacement in the Rio Pisco area started with an earlier flare-up at about 130 Ma.