U54A-01
The Acatlan Complex, Southern Mexico: Ordovician-Devonian Passive Margin on the Southern Rheic Ocean Margin and Carboniferous-Permian Subduction on Western Pangea
New structural, geochronological and geochemical data from the Acatlan Complex of southern Mexico shows it preserves a history of Cambro-Ordovician rifting along the southern margin of the Rheic Ocean, followed by Devono-Carboniferous subduction and exhumation and development of a Permo-Triassic transtensional arc on the eastern Pacific margin. The oldest rocks are represented by low-grade, siliciclastics intruded by bimodal, rift- related, mafic and felsic plutons and continental rift tholeiitic dikes ranging in age from ca.480 to 440 Ma. Greenschist facies psammites and pelites containing detrital zircons as young as ca.360 Ma represent continental rise deposits that contain tectonic slices of mafic rocks with oceanic tholeiitic and alkalic affinities. Tectonically interleaved with these greenschist facies rocks are slices of high pressure eclogite with a continental rift tholeiitic protolith, blueschist mafic rocks, high grade rift-passive margin, metasedimentary rocks, ca.470-420 Ma granitoid rocks, and periarc serpentinite. Concordant U-Pb zircon data on one of these eclogites yields an age of ca.346 Ma interpreted as the age of metamorphism that was closely followed by rapid decompression migmatization and cooling through ca.350°C by ca.350 Ma (40Ar/39Ar muscovite data). Carboniferous to Middle Permian exhumation of these high grade rocks occurred after the amalgamation of Pangea and during: (1) deposition of continental to shallow marine rocks with two pulses of fault-related conglomerates in the lower Mississippian (Kinderhookian) and Lower Permian (Leonardian); (2) listric normal deformation under greenschist facies conditions dated at ca.347 Ma and ca.300 Ma (40Ar/39Ar muscovite and amphibole data, respectively), and associated with either core complex development or extrusion; (3) Permian arc magmatism that extends along the western margin of Pangea through Mexico; and (4) Permian, dextral transtensional deformation. The western margin of Pangea during the Triassic changes from rift and passive margin in northern Mexico to S-vergent thrust and associated clastic wedge in the Acatlan Complex consistent with oblique dextral convergence between the Pacific and Pangean plates.
U54A-02
Provenance Ages of Protoliths From the Chiapas Massif Complex and Adjacent Strata of the Southern Maya Block - Implications on the Paleozoic Reconstruction of Middle America.
The basement of the Maya block is exposed in the Maya Mountains of Belize, the Chuacús Complex of Guatemala, and in the Chiapas Massif Complex (CMC) of SE Mexico. In the CMC medium- to high-grade metasedimentary rocks occur as isolated domains in mostly metaigneous crystalline rocks. The most important tectonothermal event in the entire CMC is of late Permian age, culminating in partial anatexis and the intrusion of the Chiapas batholith. In this work we present U-Pb data obtained by LA-MC-ICPMS from detrital zircon cores of metasediments from the CMC and from detrital zircons of Paleozoic strata exposed in SE Chiapas. The Pennsylvanian-Permian Santa Rosa Formation (SRF) contains mostly Pan-African (500-650 Ma) zircons, minor populations of Silurian-Early Devonian (400-420 Ma) and Grenville (1.0-1.25 Ga) zircons, and few Paleoproterozoic and Archean grains. The maximum sedimentation age is documented by ~320 Ma old zircons. Metagreywacke and metasandstones of the central CMC have inherited detrital zircon cores with age distributions indistinguishable from those of the SRF. High-grade metapelites and para-amphibolites from the CMC, instead, have inherited zircon cores with one single population of 1.0 Ga or with populations at 1.0, 1.2, and 1.5 Ga. In the southern part of the CMC leucocratic granites intrude sedimentary rocks whose detrital zircons yielded mostly 1.53 Ga ages with some grains in the range of 1.6-1.7 Ga, but no younger zircons. White mica grown in contact with the leucogranite has a 40Ar- 39Ar age of 406 ± 4 Ma, defining a minimum age for both deposition of the sediments and intrusion of the leucogranite. Our data indicate that the CMC has a composite pre-metamorphic basement, containing sedimentary protoliths from the Pennsylvanian-Permian SRF and from early Paleozoic strata intruded by Silurian-Early Devonian granites. This favors a similar pre-Permian geologic history for the CMC as for the Maya Mountains of Belize. The early Paleozoic sediments were shed from a Mesoproterozoic landmass with ages typically observed from the SW Amazon craton; however, it seems more likely that during the Silurian-Early Devonian the Maya block was situated close to the southern Grenville Province of Laurentia where ancestors from the SW Amazon craton, but no Pan-African belts, are present. By initiating the collision between Gondwana and Laurentia during the Carboniferous, the sediments were shed mostly from the newly formed orogens close to West Africa, which contains mainly Pan-African crust.
U54A-03
U-Pb zircon geochronology of Paleozoic units in Western and Central Guatemala: insights into the tectonic evolution of Middle America
Precambrian and Paleozoic basements are present in southern Mexico and Central America, where several crustal blocks are recognized by their different geologic record, and juxtaposed along lateral faults. Some of those crustal blocks are currently located between southernmost north America (the Maya block) and Central America (Chortis block).To better understand the geology of these crustal blocks, and to establish comparisons between their geologic history, U-Pb ages of both igneous and metasedimentary key units cropping out in central and western Guatemala are presented here. In the Altos Cuchumatanes (Maya block) granites yield both Permian (269 +/- 29 Ma) and Early Devonian (391 +/- 7.4 Ma) U-Pb ages. LA-ICPMS detrital zircon ages from rocks of the San Gabriel sequence, interpreted as the oldest metasedimentary unit of the Maya block, and overlain by the Late Paleozoic Upper Santa Rosa Group, yield Precambrian detrital zircons bracketed between 920 Ma and 1,000 Ma. The presence of these metasedimentary units, as well as Early Devonian to Silurian granites in the Mayan continental margin, from west (Altos Cuchumatanes), to east (Maya Mountains of Belize) indicate a more or less continuous belt of Lower Paleozoic igneous activity, also suggesting that the continental margin of the Maya block can be extended south of the Polochic fault, up to the Baja Verapaz shear zone. A metasedimentary sample belonging to the Chuacus Complex yielded detrital zircons with ages between 440 Ma and 1,325 Ma. The younger ages are similar to the igneous ages reported from the entire southern Maya continental margin, and show proximity of the Complex in the Middle-Late Palaeozoic. The S. Diego Phyllite, which overlies high-grade basement units of the Chortis block, contains zircons that are Lower Cambrian (538 Ma), Mesoproterozoic (980 to 1,150 Ma) and even Paleoproterozoic (1,820 Ma). Absence of younger igneous zircons in the San Diego Phyllite indicates that either its sedimentation took place in a close range of time, during the Late Cambrian, or absence of connection between Chortis and Maya blocks during the Early-Mid Palaeozoic. The Precambrian zircons could have come from southern Mexico (Oaxaca and Guichicovi Complexes), or from Mesoprterozoic Massifs exposed in Laurentia and Gondwana. Paleogeographic models for Middle America are limited to post-Jurassic time. The data presented here shed light on Paleozoic and, possibly, Precambrian relationships. They indicate that Maya and the Chortis did not interact directly until the Mesozoic or Cenozoic, as they approached their current position.
U54A-04
THE ACADIAN OROGENY IN THE APPALACHIAN OROGEN
The Appalachian orogen involves a number of orogenic events that reflect (i) the Cambrian-Early Ordovician foundering of the Laurentian and Gondwanan passive margins (Taconic and Penobscot orogenies, respectively), (ii) Late Ordovician-Early Silurian collision of Baltica and peri-Gondwanan terranes (e.g. Ganderia, Avalonia, Carolinia) with Laurentia (Caledonian and Salinian orogenies, respectively), events which accompanied the closure of the Iapetus Ocean and the formation of Laurussia, (iii) Siluro-Devonian Acadian Orogeny, and (iv) Permo-Carboniferous terminal collision between Laurussia and Gondwana (Alleghanian and Variscan orogenies) which gave rise to Pangea. Interpretations for the origin of the Siluro-Devonian Acadian orogeny vary from: (a) collision of Avalonia to Laurentia; (b) northward subduction of the Rheic Ocean and overriding of an oceanic plume following the collision between Laurentia and Avalonia; and (c) oblique collision of the Meguma terrane to Avalonia, which resulted in dextral transpression along the boundary. These three events had an important role in the development of the Appalachian orogen, but assignment of each event to the Acadian orogeny results in unnecessary confusion The current nomenclature is profoundly influenced by the varying views on the relationships between the peri- Gondwanan terranes, whether they traveled as separate terranes and collided with Laurentia independently, or whether some (or all) were part of a larger superterrane that became dispersed after collision. We propose a terminology that can accommodate these different viewpoints and respond to changes in interpretation as new data become available. We suggest that collision of all peri-Gondwanan terranes be referred to as the Salinic orogeny, with Ganderian, Avalonian and Carolinian stages for those who interpret these terranes to have collided independently. We suggest that the term Acadian orogeny be reserved for events related to subduction of the Rheic Ocean, with Meguma and Carolinian stages for those who believe that these terranes collided independently from within that ocean. In this scheme a separate Neo-Acadian orogeny would be unnecessary.