A MAFIC AND ULTRAMAFIC SUITE IN COASTAL GUERRERO, SOUTHERN MEXICO: EVIDENCE FOR AN EOCENE, EXTENSIONAL EPISODE RELATED TO THE RIFTING OF THE CHORTIS BLOCK?

In the south-western Guerrero state the volcano-sedimentary to low-grade metamorphic rocks of the Guerrero terrane and the low- to medium-grade metamorphic and plutonic rocks of the Xolapa terrane are intruded by Paleocene to Early Eocene continental arc granitoids. Along the coast these granitoids and Tertiary continental sediments are intruded by at least six 5 to 7 km-wide mafic intrusive bodies exposed between El Calvario beach and La Palma ranch that 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 clinopyroxene-orthopiroxene +/- olivine gabbros to horneblende-biotite diorites. They show typical arc characters, with enrichment of LILE relative to HFSE, and negative Nb-Ta anomalies. Similarly, light rare earth elements are enriched with respect to heavy rare earth elements (La/Yb = 2.59-5.55). Zircons from a diorite and one gabbro from the mafic bodies were dated by U-Pb method by the LA-ICP-MS technique, yielding a ²⁰⁶Pb/²³⁸U weighted average age of 40.6 ± 0.4 Ma and 40.8 ± 0.4 Ma respectively. 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. One tonalite intruded by one of the mafic intrusion yielded an age of 48.5 ± 0.5 Ma and the youngest detrital zircons from an intruded sandstone are ~43 Ma. Other mafic and ultramafic “alaskan-type” intrusive bodies with widespread serpentinization were previously documented in the same region (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 progress to establish if these “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.