geochemistry and u pb protolith ages of eclogitic rocks of the asis lithodeme piaxtla suite acatlan complex southern mexico tectonothermal activity along the southern margin of the rheic ocean recent data indicating that the piaxtla suite acatlan complex southern mexico underwent eclogite facies metamorphism and exhumation during the devono carboniferous suggest an origin within the rheic ocean rather than the iapetus ocean the asis lithodeme piaxtla suite consists of polydeformed metasediments and eclogitic amphibolites that are intruded by megacrystic granitoid rocks u pb zircon data indicate that the metasediments were deposited after c 700 ma and before intrusion of c 470 420 ma quartz augen granite the metasedimentary rocks contain abundant mesoproterozoic detrital zircons c 1050 1250 ma and a few zircons in the range of c 900 992 and c 1330 1662 ma their geochemical and sm nd isotopic signature is typical of rift related passive margin sediments derived from an ancient cratonic source which is interpreted to be the adjacent mesoproterozoic oaxacan complex megacrystic granites were derived by partial melting of a c 1 ga crustal source similar to the oaxacan complex amphibolitic layers exhibit a continental tholeiitic geochemistry with a c 0 8 1 1 ga source t dm age and are inferred to have originated in a rift related environment by melting of lithospheric mantle in the ordovician this rifting may be related to the early ordovician drift of peri gondwanan terranes e g avalonia from gondwana and the origin of the rheic ocean

structural geochemical and geochronological analysis of metasedimentary and metavolcanic rocks of the coatlaco area acatlan complex southern mexico abstract low grade siliciclastic metasedimentary and mafic metavolcanic rocks near coatlaco southern mexico form part of the acatlan complex which has been interpreted as a vestige of either the iapetus or rheic oceans they form two tectonically interleaved units the canoas unit consisting of interbedded psammites and pelites and the coatlaco unit made up of interbedded quartzite and tholeiitic within plate pillow basalts detrital zircons in these two units yielded different age population peaks ordovician 459 14 ma and ca 900 1250 ma detrital zircons in the canoas unit and 357 35 ma 563 22 ma 837 28 ma and 1156 74 ma in the coatlaco unit whereas a source for the paleozoic zircons may be found in the acatlan complex and provenance for the ca 900 1250 ma zircons is likely the adjacent oaxacan complex the neoproterozoic zircons probably have a more exotic source in amazonia the contrasting age populations in the canoas and coatlaco units may be due to either a different ages of deposition post 453 6 ma and post devonian respectively or b contemporaneous deposition in distinct environments continental margin vs oceanic with currents perpendicular and parallel to the margin respectively the canoas unit was deformed by four phases of deformation d1 and d2 produced ne sw steeply dipping composite axial planar solution cleavages scn1 and scn2 with variably plunging tight isoclinal folds fcn1 and fcn2 under lower greenschist facies metamorphic conditions d3 produced a sub vertical ne sw crenulation cleavage parallel to the axial planes of open to close folds whose plunges show a ne sw great circle distribution d4 produced small vertically plunging chevron and kink folds fcn4 in the pelitic rocks the coatlaco basalts are massive lack deformational fabrics and were altered under greenschist facies metamorphic conditions the coatlaco quartzites in contrast record two phases of deformation i an scl1 solution cleavage and ii an scl2 greenschist facies crenulation cleavage the less intense deformation in the coatlaco unit compared to the canoas unit is attributed to its higher competency thus scl1 2 is correlated with scn2 3 and as they post date 357 35 ma the youngest detrital zircons they probably correlate with similar carboniferous permian structures elsewhere in the acatlan complex these structures have been related to the exhumation of high pressure rocks in the acatlan complex and subsequent permian dextral movements at the western end of the variscan orogen during closure of the rheic ocean

mid jurassic tectonothermal event superposed on a paleozoic geological record in the acatlan complex of southern mexico hotspot activity during the breakup of pangea u pb isotopic analyses of zircon from the lowest structural units of the acatlan complex of southern mexico indicate that paleozoic tectonothermal events are overprinted by mid jurassic 175 3 to 171 1 ma low pressure migmatization 5 6 kb polyphase deformation and intrusion of felsic and mafic magmas ensuing rapid cooling recorded by 40ar 39ar muscovite biotite and k feldspar ages is estimated to have taken place at 21 3 c my at exhumation rates of 0 6 mm yr such rapid exhumation requires a combination of erosion and tectonic unroofing that is recorded by top to the west kinematic data synchronous tectonic unroofing is also recorded 100 km to the east in the adjacent oaxaca terrane where top to the north extensional shear zones occur in paleozoic strata this pattern of extension suggests tectonic unroofing in response to domal uplift radius 100 km like that associated with core complexes slab windows and hotspots most tectonic analyses for the jurassic place the acatlan complex in the forearc region of an arc in colombia lying 600 800 km inboard of the subduction zone presumably in response to flat slab subduction modern analogues suggest that flat slab subduction reflects subduction of young buoyant oceanic lithosphere adjacent to either a mid oceanic ridge or a plume since core complexes are typical of arc backarc regions and slab windows generally produce metamorphic belts the forearc setting and associated domal uplift suggest a plume to be the most likely cause of this jurassic tectonothermal pulse in southern mexico this plume activity is synchronous with the opening of the gulf of mexico during the breakup of pangea to which it may have contributed

ordovician silurian rift passive margin on the mexican margin of the rheic ocean overlain by carboniferous permian periarc rocks evidence from the eastern acatlan complex southern mexico abstract in the xayacatlan area of the acatlan complex in southern mexico several n s fault bounded packages of rocks occur from west to east i greenschist facies interbedded psammites and pelites huerta unit the youngest concordant detrital zircon in the western part of the unit is 455 4 ma ii greenschist amphibolite facies arkoses and psammites amate unit the youngest concordant detrital zircon gave a 206pb 238u age of 902 14 ma and older concordant ages of 900 1300 ma iii low grade clastic rocks and marbles permian tecomate formation and iv greenschist facies psammites pelites and thin tholeiitic mafic slices salada unit the youngest concordant detrital zircon yielded a 206pb 238u age of 352 3 ma and older concordant ages of 434 485 ma 511 630 ma and 920 1200 ma the eastern part of the huerta unit is cut by pegmatite that yielded an almost concordant 206pb 238u zircon age of 464 4 ma a mean age of the four almost concordant young zircons is 485 5 ma interpreted as the time of intrusion and inherited zircons with concordant ages of 530 580 ma and 910 1200 ma these data suggest that the huerta unit ranges in age from early possibly older to late ordovician the amate unit is cut by granitic dikes in which the youngest concordant 206pb 238u zircon ages are 452 6 ma and 447 3 ma interpreted as the time of intrusion and older inherited zircon ages with concordant 206pb 238u ages in the range of 950 1250 ma the intrusive ages are similar to the 442 1 ma concordant u pb age for mafic dikes that also cut the amate unit these data constrain the depositional age of the amate unit between 900 ma and latest ordovician the age of the salada unit is probably carboniferous because it is structurally overlain by the lower middle permian tecomate unit the preponderance of 510 630 ma and 900 1250 ma ages in both the detrital and inherited ages in some of the dated samples suggests a source in the adjacent and subjacent maya and oaxacan complex respectively although a xenocrystic host rock source cannot be ruled out for the intrusive rocks the igneous intrusions have been interpreted as part of a 480 440 ma bimodal rift related suite their synchroneity with deposition of the huerta unit and possibly also the amate unit suggests a rift passive margin environment of deposition on the southern margin of the rheic ocean on the other hand the salada unit and the tecomate formation appear to be synchronous with a permo triassic magmatic arc that developed on the western margin of pangea

ordovician and late paleozoic early mesozoic tectonothermal history of the la noria area northern acatlan complex southern mexico record of convergence in the rheic and paleo pacific oceans abstract the la noria area lies in the northern part of the polydeformed acatlan complex southern mexican mixteca terrane and the rocks record the following sequence of events i early middle ordovician deposition of the volcaniclastic el epazote and las calaveras units ii late middle ordovician intrusion of the 467 16 ma megacrystic peraluminous rift related granitoids iii late devonian d1 greenschist facies deformation iv intrusion of the los malpasos leucogranite and associated minor intrusions v middle mississippian d2 dextral n s deformation also under greenschist facies metamorphic conditions and vi undated d3 kink band development u pb la icpms detrital zircon ages a the el epazote unit yielded a mean 206pb 238u age from the youngest five concordant 206pb 238pb age of 488 10 ma with other age peaks at ca 506 ca 1077 and ca 1779 ma and a few concordant neoproterozoic ages and b the las calaveras unit yielded a mean 206pb 238u age from the sixteen youngest detrital zircons of 466 10 ma with other population age peaks ca 1111 and ca 1753 ma these data imply that granitoid intrusion was roughly synchronous with deposition of some of the host rocks whereas sources for most of the detrital zircons may be found in either the acatlan and oaxacan complexes laurentia or gondwana a neoproterozoic source is most likely in amazonia the rocks record three low grade deformational episodes i d1 produced a weak compositional banding and or schistosity s1 under greenschist facies conditions ii d2 also occurred under greenschist facies conditions and developed tight to isoclinal folds f2 in s1 and an axial planar spaced cleavage s2 that is co planar with s1 and iii d3 produced reverse and conjugate kink bands f3 that deform the s1 s2 composite foliation the leucogranite and related dikes that intrude the complex record only the latter two deformational events ca 330 ma 40ar 39ar muscovite plateau ages probably closely post date the d2 event d1 may be correlated with early carboniferous deformation elsewhere in the acatlan complex on the other hand the initial 40ar 39ar steps at ca 300 220 and 172 ma probably indicate thermal disturbances below 300 c during the permo carboniferous triassic and jurassic respectively whereas the ordovician history of the plutons and volcano sedimentary units coincides with the lifespan of both the iapetus and rheic oceans the late paleozoic early mesozoic deformation better reflects closure of the rheic ocean and convergence tectonics on the paleo pacific margin following the amalgamation of pangea

pressure temperature and time indicators in mafic schist their application to reconstructing the polymetamorphic history of vermont mineral chemistry and overgrowth relationships in mafic schist from vermont are shown to be sensitive indicators of pressure temperature and relative time and to be good chronicles of the paleozoic history of this polymetamorphic terrane within the common assemblage amphibole chlorite epidote plagioclase quartz ti phase carbonate k mica fe 3 oxide electron microprobe analyses show that increasing metamorphic grade as defined by intercalated pelitic schist is recorded by an increase in the edenite glaucophane and tschermakite contents of amphibole in the anorthite content of plagioclase and in the substitution of al vi f 2 ti al iv for fe 2 mg mn si in biotite chlorite and muscovite with increasing pressure the glaucophane component of amphibole increases for medium pressure metamorphism the albite oligoclase gap is in the garnet zone where amphibole has between 1 2 and 1 8 formula proportion al iv and al vi fe 3 ti cr this gap is below the garnet isograd in low pressure mafic schist where al iv and al vi fe 3 ti cr in the amphibole are both less than 0 6 mineral growth periods observed are characterized by metamorphic grade and facies series and are assigned to two ordovician taconic and two devonian acadian events silurian devonian mafic schist in northeastern vermont records two periods of low pressure devonian metamorphism these events are expressed by medium pressure and low pressure metamorphism in the cambrian and ordovician rocks to the west and south in the pre silurian rocks high pressure and medium pressure metamorphism is assigned to the ordovician the observed high pressure metamorphism is confined to a 110 by 40 km area along the green mountain anticlinorium axis in north central vermont

disordering of 13c18o bonds in co2 gas over a heated quartz surface at 50 1100 c insights into the abundance of mass 47 47 in co2 gas at thermodynamic equilibrium abstract the clumped isotope composition or the 47 value of co 2 relates the abundance of 13 c 18 o 16 o among all the co 2 isotopologues to the formation temperature of a carbonate mineral or the equilibration temperature of co 2 gas the 47 data typically requires several standardization steps and one of which heavily relies on true or equilibrium 47 values from different temperatures however they have not yet been experimentally quantified and only theoretically predicted 47 values are available to the community e g wang et al 2004 in this study a series of co 2 gases were cleaned using a custom designed automated co 2 cleaning system baked in a pre cleaned dry quartz tube and then measured for their 47 values the cbm protocol to examine 47 change kinetics of co 2 gas over a heated quartz surface between 50 and 1100 c in another test a series of co 2 gases were baked first cleaned subsequently and finally measured for their 47 values the bcm protocol to evaluate the effect of the co 2 cleaning on the 47 value in a routine 47 measurement our experimental results showed that 47 value of the co 2 gas over a heated quartz surface attained thermodynamic equilibrium in 191 7 0 23 and 0 1 h at 50 100 491 and 1000 c respectively we also reported the first experimentally determined equilibrium 47 t relation in co 2 gas between 50 and 1100 c as follows 47 in t in c r 2 0 99 p absolute 47 0 9791 0 0029 e 0 0045 t the comparison between the two co 2 preparation protocols also revealed that the co 2 cleaning step after the baking in the bcm protocol could alter the original 47 signature of co 2 gas by 0 04 and 0 07 at 100 and 1000 c respectively increasing the empirical transfer function etf slope to 1 therefore the isotope scrambling effect reported in the literature e g dennis et al 2011 might be resulted from co 2 cleaning process because isolating the co 2 cleaning effect before the baking yielded an etf slope of 1 implying that instrumental or analytical artifacts that could affect the measured 47 were not significant we suggest using a set of heated co 2 gases prepared by the bcm protocol from several different temperatures as an alternative for the conventional use of heated and water equilibrated co 2 gases to construct an absolute reference frame and standardize 47

an empirical evaluation of the argon diffusion geometry in muscovite the temperature time significance of 40ar 39ar muscovite ages is difficult to evaluate quantitatively because of ambiguities in the choice of an appropriate model diffusion geometry the effective diffusion dimension and large uncertainties in experimentally determined diffusion parameters laser 40ar 39ar studies of natural 40ar concentration gradients suggest that argon loss in muscovite occurs primarily via cylindrical volume diffusion commonly with an effective diffusion dimension equal to the crystal diameter the present study tests the cylinder model geometry and the control of physical grain size on the effective diffusion dimension by measuring the anisotropy of a natural40ar diffusion gradient parallel and perpendicular to 001 in a single euhedral muscovite phenocryst the new data combined with previous studies clearly indicate loss of 40ar via cylindrical volume diffusion and strongly suggest that loss of 40ar across the 001 sheets slab diffusion geometry is minimal in undeformed crystals

acatlan complex southern mexico record spanning the assembly and breakup of pangea new structural geochronological and geochemical data from the acatlan complex of southern mexico show that it preserves a complete history of pangea from assembly to breakup previously interpreted to be a vestige of the iapetus suture the acatlan complex records a history that can be sequentially linked to the rheic ocean the paleo pacific and the gulf of mexico this record is interpreted to reflect 1 the development of a rift passive margin on the southern flank of the rheic ocean in the cambrian ordovician 2 the formation of an extensional regime along the formerly active northern margin of gondwana throughout the ordovician 3 closure of the rheic ocean documented by subduction related eclogite facies metamorphism and exhumation during the late devonian mississippian 4 permian triassic convergent tectonics on the paleo pacific margin of pangea and 5 interaction with a jurassic mantle plume coeval with the opening of the gulf of mexico

pressure temperature time evolution of high pressure rocks of the acatlan complex southern mexico implications for the evolution of the iapetus and rheic oceans comment vega granillo et al 2007 1 are to be congratulated for publishing high quality pressure temperature time p t t data on the high pressure hp rocks within the acatlan complex fig 1 2 these data expand on previously published p t data of meza figueroa et al 2005 3 the

late ordovician early silurian continental collisional orogeny in southern mexico and its bearing on gondwana laurentia connections new zircon and monazite u pb data tectonic mapping and petrologic studies in key units of the acatlan complex show a previously undocumented phase of continental collision orogeny of late ordovician early silurian age in southern mexico the event involved the partial eclogitization of oceanic lithosphere and continental crust which traveled westward more than 200 km over siliciclastic metasedimentary rocks of the trench forearc of an opposing continental margin the overriding eastern margin was the oaxaquia microplate attached to gondwana and the western overridden margin is considered to have been the eastern margin of laurentia this event which we name the acatecan orogeny was roughly synchronous with the possible closure of iapetus along the appalachian margin which involved according to current models either the docking of peri gondwanan terranes such as avalonia and carolina or the direct collision between gondwana and laurentia the permanence of oaxaquia in northwestern gondwana until the end of the silurian as suggested by tremadocian to silurian marine faunas in the cover of oaxaquia is more consistent with the direct collision of gondwana and laurentia at the end of the ordovician forming the acatlan complex between

pressure temperature time evolution of high pressure rocks of the acatlan complex southern mexico implications for the evolution of the iapetus and rheic oceans replyreply to comment by keppie et al

laser probe 40 ar 39 ar ages of early and late stage alteration assemblages rosario porphyry copper molybdenum deposit collahuasi district i region chile sensibly identical laser probe 40 ar 39 ar dates of 32 9 or 0 5 2sigma and 32 9 or 0 6 ma are determined for respectively early stage hydrothermal biotite and late stage hydrothermal muscovite from the giant rosario porphyry copper deposit in the collahuasi district of northern chile the age determinations were carried out on ca 1 cm 2 1 to 0 5 mm thick wafers of altered rock or on smaller chips broken therefrom obviating the requirement for mineral separation and permitting direct petrographic control of the analytical procedure the age data although not unambiguous imply that this extremely large hydrothermal center was emplaced over a short interval hydrothermal activity in this central andean transect migrated eastward over a period of ca 3 to 4 m y from copaquire through quebrada blanca to rosario the intensity of mineralization increasing with time rosario chuquicamata and la escondida the largest of the west fissure domeyko fault system array were apparently emplaced coevally in the terminal stage of the late eocene early oligocene metallogenic episode

conditions of high grade metamorphism in the dalradian of connemara ireland in e connemara ireland dalradian metasediments were initially metamorphosed to kyanite staurolite grade at pressures above 7kbar the subsequent emplacement of basic to intermediate magmas accompanied by uplift and erosion produce a steepening of the geothermal gradient and led to the formation of migmatitic garnet cordierite sillimanite gneisses in the highest grade regions in the s cation exchange equilibria between coexisting mineral pairs suggest that partial melting took place under high temperatures 750 c and moderate pressures 4 5 6 kbar fluid inclusion studies and andalusite occurrences from migmatite leucosomes suggest that crystallization of the melts occurred at lower pressures

guidelines for assessing the provenance of mesozoic and cenozoic clastic successions sourced by pre jurassic basement complexes in southernmost north america

early mesozoic southern mexico amazonian connection based on u pb ages from detrital zircons the la mora paleo river in the mixteca terrane and its paleogeographic and tectonic implications abstract the la mora formation is the oldest mesozoic floodplain succession in the mixteca terrane of southern mexico the presence of amazonian detrital zircons in the la mora formation and in the overlying volcanic diquiyu unit indicates a major fluvial system that drained the mixteca terrane the la mora paleo river crossed the oaxaquia microcontinent and the mixteca terrane prior to the breakup of pangea during late triassic early jurassic time when the acatlan oaxaquia block was part of the northwestern portion of the amazonian craton detrital zircons in the la mora formation have ages between 3307 31 and 210 12 ma which suggest that they originated in amazonia and southern mexico 33 2 of the detrital zircons have ages that are found only in amazonian sources whereas 66 7 of the zircons may be associated with either the amazonian craton the andean basement or southern mexico we propose that the la mora fluvial system drained the amazon basin in a westward direction with its mouth in central pangea and that it most likely fed the toliman submarine fan the inferred location of the acatlan oaxaquia tectonic block at the time of the la mora fluvial system implies that the basement of southern mexico experienced a right lateral displacement of at least 2300 km with respect to south america during the mesozoic

extensional late paleozoic deformation on the western margin of pangea patlanoaya area acatlan complex southern mexico abstract new mapping in the northern part of the paleozoic acatlan complex patlanoaya area records several ductile shear zones and brittle faults with normal kinematics previously thought to be thrusts these movement zones separate a variety of units that pass structurally upwards from i blueschist eclogitic metamorphic rocks piaxtla suite and mylonitic megacrystic granites columpio del diablo granite ordovician granites elsewhere in the complex ii a gently e dipping listric normal shear zone with top to the east kinematic indicators that formed under upper greenschist to lower amphibolite conditions iii the middle late ordovician las minas quartzite upper greenschist facies psammites with minor interbedded pelites intruded by mafic dikes and a leucogranite dike from the columpio del diablo granite unconformably overlain by the otate meta arenite lower greenschist facies psammites and pelites roughly temporal equivalents are the middle late ordovician mal paso and ojo de agua units interbedded metasandstone and slate and metapelite and mafic minor intrusions respectively some of these units are intruded by the massive 461 2 ma palo liso megacrystic granite decussate contact metamorphic muscovite yielded a 40 ar 39 ar plateau age of 440 4 ma iv a steeply moderately e dipping normal fault v latest devonian middle permian sedimentary rocks patlanoaya group here elevated from formation status the upward decrease in metamorphic grade is paralleled by a decrease in the number of penetrative fabrics which varies from i three in the piaxtla suite through ii two in the las minas unit e trending sheath folds deformed by ne trending subhorizontal folds with top to the southeast asymmetry both associated with a solution cleavage iii one in the otate mal paso and ojo de agua units steeply se dipping ne sw plunging open close folds to iv none in the patlanoaya group 40 ar 39 ar analyses of muscovite from the earliest cleavage in the las minas unit yielded a plateau age of 347 3 ma and show low temperature ages of 260 ma post dating all of these structures and the patlanoaya group are ne plunging subvertical folds and kink bands an e w vertical normal fault juxtaposes the low grade rocks against the anacahuite amphibolite that is cut by megacrystic granite sheets both of which were deformed by two penetrative fabrics amphibole from this unit has yielded a 40 ar 39 ar plateau age of 299 6 ma which records cooling through 490 c and is probably related to a permo carboniferous reheating event during exhumation the extensional deformation is inferred to have started in the latest devonian 360 ma during deposition of the basal patlanoaya group lasting through the rapid exhumation of the piaxtla suite at 350 340 ma synchronous with cleavage development in the las minas unit deposition of the patlanoaya group with active fault related exhumation suggested by mississippian and early permian conglomerates 340 and 300 ma respectively and continuing at least into the middle permian 260 ma muscovite ages the continuity of mid continent mississippian fauna from the usa to southern mexico suggests that this extensional deformation occurred on the western margin of pangea after closure of the rheic ocean

ordovician earliest silurian rift tholeiites in the acatlan complex southern mexico evidence of rifting on the southern margin of the rheic ocean abstract the acatlan complex of southern mexico is a vestige of a paleozoic ocean inferred to be either the cambro ordovician iapetus and or the ordovician carboniferous rheic oceans ordovician granitoids in the complex have been interpreted as either the products of dehydration melting arc or rift magmatism however the geochemistry of felsic rocks is inconclusive the geochemistry of a recently discovered major ordovician earliest silurian mafic igneous suite associated with these granitoids is critical to defining the tectonic setting of the igneous event and to paleogeographic reconstructions such data from three areas in the acatlan complex xayacatlan patlanoaya and cuaulote document tholeiitic within plate characteristics with a source in either primitive mantle or mantle previously modified by subduction related magmatism possibly combined with crustal contamination this combined with their occurrence as a dike swarm intruding rift passive margin clastic sedimentary rocks indicates rifting of a continental margin mafic dikes at xayacatlan yielded a concordant u pb tims zircon age of 442 1 ma and a 40 ar 39 ar hornblende plateau age of 434 3 ma the age of mafic magmatism at other localities is defined by the ages of associated granitoids intruded at ca 461 ma and by the age of the youngest detrital zircon in the host rocks 496 25 ma at patlanoaya previously published age data suggest that this igneous event may have started earlier at 478 5 ma early ordovician although the life spans of the iapetus and rheic oceans overlap during the ordovician subduction and collision characterize the former whereas the latter was in a rift drift mode thus this ordovician earliest silurian magmatism is more consistent with rifting along the southern margin of the rheic ocean initiation of rifting at ca 480 ma is contemporaneous with separation of peri gondwanan terranes such as avalonia and carolinia from amazonia oaxaquia subsequent rifting may be analogous to mesozoic cenozoic western north america where transtensional deformation is related to dispersal of terranes e g baja california and cretaceous baja british columbia leading to a gradual collapse of the inboard continental margin

a volcanic degassing event at the explosive effusive transition 1 a sequence of five vulcanian explosions followed a lava dome collapse in july 2003 at soufriere hills volcano each explosion occurred at t 190 n4 3 where n 1 5 and t is the time s since the decompression rate peak during the collapse instead of a sixth explosion at the predicted time a rapid emission of 97 103 kg so2 was observed by a spectrometer network this event represents the transition from explosive to effusive activity after the last explosion high magma ascent rates were maintained but the critical overpressure explosion criterion was not reached instead degassing and crystallisation in the upper conduit caused horizontal gradients in viscosity and flow rate and brittle failure at the walls when the rate of shear strain exceeded a critical value development of a permeable shear zone allowed gas release relief of overpressure and a return to effusive lava dome building

pressure temperature time evolution of paleozoic high pressure rocks of the acatlan complex southern mexico implications for the evolution of the iapetus and rheic oceans new thermobarometric and u pb and 40 ar 39 ar geochronologic data coupled with ages obtained from the acatlan complex the basement of the mixteco terrane of southern mexico reveal the existence of three distinctive high pressure metamorphic events of early to middle paleozoic age each recorded in a separate lithological suite xayacatlan suite eclogites with oceanic affinity underwent peak metamorphism at 609 491 c and 13 12 kb during the early ordovician ca 490 477 ma u pb zircon followed by a partial overprint at 600 c and 9 6 kb and then at 500 c and 6 7 kb an overprinting event at 525 500 c and 9 5 kb is ascribed to the devonian the pressure temperature p t path of the xayacatlan suite indicates a subduction exhumation process followed by tectonically related reburial ixcamilpa suite blueschists with oceanic affinity underwent epidote blueschist metamorphism t 200 390 c p 6 9 kb and then epidote amphibolite t 390 580 c p 9 6 kb events ascribed to the late ordovician early silurian esperanza suite eclogites with continental affinity underwent peak metamorphism at 830 730 c and 17 15 kb amphibole from eclogite yields a 430 5 ma 40 ar 39 ar age dating the high pressure hp event p t paths of high temperature ht eclogites like those of the esperanza suite have been related to the collision of continental blocks partial overprinting occurred at 690 640 c and 14 10 kb prior to 374 2 ma 40 ar 39 ar phengite the three hp suites were tectonically juxtaposed at different times before the mississippian period resulting in the closure of the iapetus ocean phengite 40 ar 39 ar geochronology reveals the existence of a widespread tectonothermal event between 345 and 323 ma which may be related to the juxtaposition of the hp composed block and the gondwanan affinity cosoltepec suite causing the closure of the rheic ocean the tectonothermal events in the acatlan complex coincide in time physical conditions and tectonic setting with events in the appalachian caledonian orogen suggesting their relation on that basis the geology of the acatlan complex can lead to a more comprehensive understanding of the tectonic evolution of the appalachian orogen and of the gondwana laurentia interactions preceding the pangean assembly

caltepec fault zone an early permian dextral transpressional boundary between the proterozoic oaxacan and paleozoic acatlan complexes southern mexico and regional tectonic implications 1 the tectonic boundary between the grenville age oaxacan and paleozoic acatlan crystalline complexes in southern mexico named the caltepec fault zone cfz is characterized for the first time as a dextral transpressional nnw trending and ene dipping ductile fault zone of early permian age the complexes are welded by a syntectonic magmatic epidote bearing granite along the entire length of the cfz from east to west the 2 6 km wide cfz consists of disrupted and retrograded banded gneisses of the oaxacan complex quartz feldspar mylonite and the syntectonic magmatic epidote bearing cozahuico granite czg with huge xenoliths up to several kilometers long and up to 600 m wide of the proterozoic gneisses thrust westward over metasedimentary tectonites of the acatlan complex the czg shows magmatic fabrics that represent a transition to solid state deformation characterized by subvertical foliation subhorizontal nne and sse dipping mineral stretching lineation and dextral kinematics the megaxenoliths underwent partial melting developing banded migmatites with layers of epidote bearing granitic neosome the parallelism of fabrics in these anatexitic rocks and in the enclosing deformed granite suggests that ductile deformation migmatization of xenolithic gneisses and granite emplacement along the cfz were coeval the neosome yielded a u pb zircon concordant age of 275 6 1 ma probably dating the peak of the tectonothermal event we interpret the cfz as a major terrane boundary accommodating transpressional interaction between the acatlan and oaxaquia blocks which were amalgamated in an oblique convergent setting by early permian time as the leading edge of gondwana impinged onto the southern margin of laurentia along the marathon ouachita suture to form pangea

geology and geochronology of paleozoic rocks in western acatlan complex southern mexico evidence for contiguity across an extruded high pressure belt and constraints on paleozoic reconstructions the acatlan complex straddles a high pressure belt previously interpreted as either 1 a suture zone within the iapetus or the rheic oceans which would have a contrasting geological record across the suture or 2 a tectonic slice extruded into the upper plate which would imply contiguity across the complex distinguishing between these hypotheses is critical to paleogeographic reconstructions examination of the western acatlan complex reveals the following 1 deposition of clastic rocks between 654 and 464 ma 2 intrusion of bimodal ordovician bodies at ca 464 ma 3 high grade deformation with cooling through 400 c by 360 335 ma 4 deposition of clastic rocks and pillow lavas after 350 400 ma 5 deformation accompanied by greenschist facies metamorphism at ca 335 ma 6 deposition of clastic and bimodal volcanic rocks at ca 327 ma 7 320 270 ma subgreenschist deformation 8 deposition of the middle upper permian sedimentary rocks and 9 intrusion of a 61 1 ma diorite followed by early cenozoic laramide ene folding and faulting zircon ages 350 400 570 505 827 890 ma 0 9 1 3 ga suggest both local and amazonian sources with deposition above a local mesoproterozoic oaxacan basement on the southern margin of the rheic ocean this geological record is very similar to that of the eastern acatlan complex which supports the extrusion hypothesis a model that may be applicable to other orogens

990 and 1100 ma grenvillian tectonothermal events in the northern oaxacan complex southern mexico roots of an orogen inliers of f1 0 1 3 ga rocks occur throughout mexico and form the basement of the oaxaquia microcontinent in the northern part of the largest inlier in southern mexico rocks of the oaxacan complex consist of the following structural sequence of units from bottom to top which protolith ages are 1 huitzo unit a 1012f12 ma anorthosite mangerite charnockite granite amcg suite 2 el catro n unit z1350 ma orthogneiss migmatized at 1106f6 ma and 3 el marquez unit z1140 ma para and orthogneisses these rocks were affected by two major tectonothermal events that are dated using u pb isotopic analyses of zircon a the 1106f6 ma olmecan event produced a migmatitic or metamorphic differentiation banding folded by isoclinal folds and b the 1004 978f3 ma zapotecan event produced at least two sets of structures z1 recumbent isoclinal class 1c 3 folds with gently nw plunging fold axes that are parallel to mineral and stretched quartz lineations under granulite facies metamorphism and z2 tight upright subhorizontal wnw to nne trending folds accompanied by development of brown hornblende at upper amphibolite facies metamorphic conditions cooling through 500 jc at 977f12 ma is documented by 40 ar 39 ar analyses of hornblende fold mechanisms operating in the northern oaxacan complex under zapotecan granulite facies metamorphism include flexural and tangential longitudinal strain accompanied by intense flattening and stretching parallel to the fold axes subsequent phanerozoic deformation includes thrusting and upright folding under lower grade metamorphic conditions the zapotecan event is widespread throughout oaxaquia and took crustal rocks to a depth of f25 30 km by orogenic crustal thickening and is here designated as zapotecan orogeny modern analogues for zapotecan granulite facies metamorphism and deformation occur in middle to lower crustal portion of subduction and collisional orogens contemporaneous tectonothermal events took place throughout oaxaquia and in various parts of the genvillian orogen in laurentia and amazonia d 2003 elsevier science b v all rights reserved