The El Calvario-La Palma Mafic and Ultramafic Suite, Coastal Guerrero: Evidence for a Major Eocene Extensional Episode in Southern Mexico?

The western coast of Guerrero, between Zihuatanejo and Acapulco, preserves a large variety of lithologic types that includes volcanoclastic, volcanic, subvolcanic, intrusive, ultramafic, and low- to medium-grade metamorphic rocks traditionally explained by the juxtaposition of two different terranes: the volcano-sedimentary to low-grade metamorphic Guerrero terrane to the west, and the low- to medium-grade metamorphic and plutonic Xolapa terrane to the east. These pre-Tertiary basement units are intruded by Paleocene to Early Eocene continental arc granitoids. One intriguing feature of this region is the occurrence of at least six, 5 to 7 km-wide mafic intrusive bodies exposed between El Calvario beach and La Palma ranch. These bodies cut the volcano-sedimentary rocks of the Guerrero terrane as well as the plutonic and metamorphic rocks of the Xolapa terrane, and produce a 500 to 100 m-thick contact aureole, with the highest temperature rocks represented by garnet-clinopyroxene hornfels. The mafic bodies vary in composition from olivine-orthopyroxene-clinopyroxene gabbros to horneblende-biotite diorites. Myrmekitic structures are very common in plagioclase at the submillimeter-scale, suggesting a shallow emplacement of magma. The N-MORB normalized multi-element patterns show typical arc characters, with enrichment of the large ion lithophile elements (LILE) relative to the high field strength elements (HFSE), and negative Nb-Ta anomalies. Similarly, light rare earth elements (LREE) are enriched with respect to heavy rare earth elements (HREE) (La/Yb = 2.59-5.55). Zircons separated from one granite intruded by one of the mafic intrusion and two diorite samples associated with the mafic bodies were dated by U-Pb method by the LA-ICP-MS technique. Analyzed grains define a 206Pb/238U weighted average age of 72.4 ± 1.1 Ma for the granite and of 40.6 ± 0.4 Ma and 40.8 ± 0.4 Ma for the diorites. Plagioclase from a gabbro of another mafic body was dated by 40Ar-39Ar method at 37.2 ± 0.3 Ma, an age consistent with a normal cooling rate for these plutonic bodies. Other mafic and ultramafic “alaskian-type” intrusive bodies with widespread serpentinization were previously documented in the El Tamarindo, Loma Baya and Puerto Vicente Guerrero, with a 40Ar-39Ar age of ~112 Ma for the former body. More geochronologic and geochemical studies are in process to establish if the “alaskian-type” intrusives are equivalent to the gabbro-dioritic suite of El Calvario-La Palma and therefore they define a single mafic-ultramafic magmatic pulse of Eocene age. In any case the shallow emplacement, as well as the primitive signature of these rocks point to a rapid ascent of these magmas, likely promoted by a strong extensional regime. This, in turn, might be related to the crustal thinning that accompanied the detachment of the Chortis block from southern Mexico.