GEOCHRONOLOGIC CONSTRAINTS ON THE EXHUMATION OF THE CHORTÍS AND MAYA BLOCKS, CENTRAL GUATEMALA

Two adjacent serpentinite mélange belts along the Motagua fault zone (MFZ) in central Guatemala contain HP/LT metamorphic blocks. South of the MFZ, the serpentinite mélange belt occurs in the Chortís block and HP/LT metamorphism was reached at ~130 Ma. Whereas north of the MFZ and south of the Polochic fault zone (PFZ), the serpentinite mélange occurs with fragments of both the Maya and the Chortís blocks and records a thermal event at ~65 Ma.

The southern serpentinite mélange has blocks of lawsonite eclogite, jadeitite and blueschist. These HP blocks in the serpentinite mélange indicate rapid exhumation after subduction. The oldest rocks in the Chortís block are the San Diego phyllite; two sandstone samples yielded detrital zircon ranging from 321 to 2895 Ma. The oldest ages tie the Chortís block to Mexico or southern California, whereas the youngest ages support a proposed Permian depositional age. The Jurassic El Tambor Fm. was metamorphosed to lower amphibolite facies at 150 Ma (Ar/Ar amphibole). The Las Ovejas Fm. records differential cooling from north to south between 30 and 50 Ma (Ar/Ar white mica). Apatite fission track geochronology indicates diachronous cooling from west to east between 24-15 Ma. Thus, the basement records a slower cooling as well as a different PT path from the serpentinite mélange.

The northern serpentinite mélange has epidote eclogite, jadeitite and garnet amphibolite. The eclogite records the 130 Ma subduction event. However, the PTt path for most of the basement as well as garnet amphibolite and jadeitite shows rapid cooling between amphibole and white mica Ar/Ar ages of ~65 Ma. Two samples of Chuacus basement near the contact with serpentinite mélange have white mica ages of ~48 Ma. Apatite fission track ages indicate final exhumation at 35-31 Ma.

Taken together, the regional geology and our geochronologic data suggest two events: subduction as the Chortís block collided with Mexico and an ocean-vergent fold and thrust belt developed, followed by a continent-vergent fold and thrust belt as the Chortís block collided with the Maya block. Exhumation of the HP/LT rocks occurred in three phases: (1) plate-boundary parallel stretching resulting from oblique convergence along a curved arc; (2) boundary parallel extension in Andaman Sea type pull-apart basins; and (3) crustal thrusting and erosion.