the evidence for middle wisconsin peopling of beringia an evaluation abstract broken large mammal fossil bones in eastern beringia have been used to argue for a middle wisconsin or earlier time of human entry into north america it has been inferred that these controversial pleistocene bones are artifacts because 1 they are similar to those found in archaeological sites and 2 they can be reproduced in the laboratory by replication however fractured flaked polished faceted cut and scratched bones similar to the purportedly human artifacts described from beringia are known to be produced by natural processes new experimental data demonstrate that identifications of pleistocene bone artifacts on the basis of differential staining and fresh breakage are suspect physical processes such as the violent forces of river ice breakups in the north and bone crushing by mammalian scavengers produce similar pseudo artifacts additionally the middle wisconsin dates associated with some of the finished tools and human and dog bones from beringia are open to question the lack of credible dates on these artifacts raises doubt about the model of human colonization of beringia developed in the 1960s which proposed an interstadial colonization of the new world

crustal scale block tilting during andean trench parallel extension structural and geo thermochronological insights despite a long history of plate convergence at the western margin of the south american plate that has been ongoing since at least the early paleozoic the southern peruvian fore arc displays little to no evidence of shortening in the light of this observation we assess the deformation history of the southern peruvian fore arc and its geodynamic implications to accomplish this we present a new structural and geo thermochronological data set zircon u pb mica 40ar 39ar apatite and zircon fission track and zircon u th he analyses for samples collected along a 400 km long transect parallel to the trench our results show that the mesoproterozoic gneissic basement was mainly at temperatures 350 c since the neoproterozoic and was later intruded by jurassic volcanic arc plutons along the coast a peculiar apatite fission track age pattern coupled with field observations and a synthesis of available geological maps allows us to identify crustal scale tilted blocks that span the coastal peruvian fore arc these blocks bounded by normal faults that are orthogonal to the trench suggest post 60 ma trench parallel extension that potentially accommodated oroclinal bending in this region block tilting is consistent with the observed and previously described switch in the location of sedimentary sources in the fore arc basin our data set allows us to estimate the cumulative slip on these faults to be less than 2 km and questions the large amount of trench parallel extension suggested to have accommodated this bending

sapphirine bearing orthopyroxene kyanite sillimanite granulites from south harris nw scotland evidence for proterozoic uht metamorphism in the lewisian sapphirine bearing orthopyroxene kyanite opx ky and sillimanite opx sil granulites have been found in the lewisian complex of south harris in northwest scotland in the opx ky granulites orthopyroxene and kyanite are intergrown in a stable mineral assemblage which indicates metamorphic condition at 800 900 c 12 kbar sillimanite inclusions within orthopyroxene suggest that sillimanite formed earlier conditions are estimated at 950 30 c at 10 kbar from orthopyroxene isopleths for aluminous orthopyroxene 8 kbar according to the fmas petrogenetic grid and similar conditions were obtained by using orthopyroxene garnet geothermobarometers the two types of orthopyroxene aluminosilicate granulites indicate that the peak metamorphic conditions were over 900 c compatible with ultra high temperature metamorphism as accessory sapphirine occurs in several assemblages and with different compositions it is interpreted to be formed at different stages of the metamorphism these granulites were formed during early proterozoic high grade metamorphism due to the emplacement of the south harris igneous complex at c 2170 1870 ma and are not related to the major metamorphic episode of the badcallian inverian metamorphism at c 2700 2500 ma in the mainland lewisian

the proterozoic of eastern bolivia and its relationship to the andean mobile belt abstract the previously unmapped bolivian sector of the central brazil shield 220 000 km in area has been studied during an anglo bolivian technical co operation programme the metamorphic basement comprises the lomas maneches granulitic complex the chiquitania gneissic complex and the san ignacio schist supergroup which crops out as metasedimentary belts rb sr dates on some of the granulites suggest a protolith age of 2000 ma but the outcrop pattern of the metamorphic rocks as a whole is due to the 1300 ma san ignacio orogeny which was accompanied by widespread granite emplacement and metamorphism up to granulite facies the sunsas cycle began with the deposition of the supracrustal sunsas vibosi group and closed with the development of the sunsas and aguapei mobile belts at 1000 ma the upper proterozoic brasiliano cycle is represented by a bifurcating sedimentary basin in the se of the area with a closing tectonic event of 520 ma correlation is suggested with units in the central brazil shield the bolivian belts are extrapolated east into brazil and their limits indicated to the west the belts show trends parallel to the phanerozoic andean chain and they reflect the major deflection in the chain the arica elbow furthermore when the belts are traced towards the andes their ages broadly correspond to those of the sub andean precambrian basement this leads to the proposal of an enlarged andean orogenic belt system 1000 km wide comprising a 1300 ma old proto andean orogen which has controlled the trends of subsequent mobile belts

geochronology and nd isotopic data of grenville age rocks in the colombian andes new constraints for late proterozoic early paleozoic paleocontinental reconstructions of the americas abstract new u pb zircon crystallization ages and 40ar 39ar cooling ages from the colombian andes confirm the existence of rocks metamorphosed during the orinoquian orogenic event ca 1 0 ga of northern south america end t 1 1 ga for these rocks range from 3 9 to 0 91 which is interpreted as a mixture of late archean early proterozoic crust with juvenile material produced during the 1 1 ga orogenic event the colombian grenville age rocks are part of a much longer metamorphic pericratonal belt sporadically exposed along the andes in western central peru southern bolivia and northern argentina in addition nd model tdm ages for the colombian rocks range from 1 9 to 1 45 ga similar to those obtained in the grenville province of the eastern u s and in the mexican basement placing constraints on late proterozoic early paleozoic paleocontinental reconstructions

counterclockwise rotation of late eocene oligocene fore arc deposits in southern peru and its significance for oroclinal bending in the central andes the results of a paleomagnetic study along the fore arc of southern peru 15 18 s and northern chile 18 19 s are reported from middle to late miocene ignimbrites 7 sites late oligocene to early miocene ignimbrites 72 sites paleogene sediments 20 sites and mesozoic and paleocene volcanics and intrusions 31 sites comparison of locality mean directions with expected paleomagnetic directions indicates vertical axis rotations ranging from 5 2 11 3 clockwise to 55 6 7 0 counterclockwise spatially the magnitude of counterclockwise rotations increases northward from 0 within the chilean fore arc south of 18 30 s to 45 north of 16 30 s in southern peru paleomagnetic rotations recorded in paleogene red beds decrease from late eocene to late oligocene whereas miocene ignimbrites display no evidence of rotation these new results confirm that the rotations recorded in the fore arc of southern peru were acquired at least before 15 ma and probably before 25 ma and thus prior to the late neogene shortening of the sub andes the onset of major andean shortening in the eastern cordillera during the latest eocene earliest oligocene is interpreted to have triggered the bending of the peruvian fore arc the region of the peruvian fore arc with the largest rotations appears to be the fore arc counterpart of the abancay deflection a remarkable ne sw offset in the axis of the eastern cordillera induced by a major regional preorogenic structure we underline that the abancay deflection should be seen as the northwestern boundary and therefore as a key element of the bolivian orocline

underpinning based simultaneous extraction of contiguous sections of a thick coal seam under weak and laminated parting around 12 5 m thick zero seam of chirimiri colliery underneath bartunga hill of m s south eastern coalfield limited secl was found in two sections 3 5m top and 6 0m bottom section with a highly laminated 3 0 m thick parting between the two sections the laminated parting comprised of alternate layers of shale carbonaceous shale coal and mudstone was found to be incompetent for safe and clean underground working of both sections both top and bottom sections were developed on superimposed pillars along the floor using roof bolts based on laboratory testing and investigations on simulated models an idea of underpinning was conceived for simultaneous depillaring of both the contiguous sections taking advantage of presence of openings in the top section the laminated parting and roof coal band of bottom section was reinforced simultaneously by a full column grouted cable bolt of suitable length from the floor of the top section underpinning consolidated parting stability through reinforcement and provided additional thickness to the critical parting thickness as the roof coal band of the thick bottom section is stitched together with the parting maintaining superimposition of workings in the two sections pillars of both the sections were extracted by splitting and slicing splits and slices of the bottom section did not experience roof instability problems due to the presence of the reinforced overlying roof coal band and parting by underpinning the roof coal of the bottom section was blasted down on retreat consolidation of the multi layered parting by underpinning did not allow separation and collapse of its mudstone shale layers resulting in clean coal blasted from the roof underpinning based simultaneous depillaring of the contiguous sections proved its technological feasibility and success through field trials

river ice as a taphonomic agent an alternative hypothesis for bone artifacts abstract the annual freezeup and violent breakup of temperate and high latitude rivers produces unique geomorphic features that can be recognized in ancient sediments the forces and materials involved in the breakup process could modify entrained bones in ways similar to those attributed to human activity an intuitive taphonomic model is developed to explain how river ice may affect bones and to predict the modifications expected partial testing of the model by experiments designed to simulate river breakup produced bone modifications such as fractures flakes facets striations isolated cuts and polish the taphonomic effects of river ice must be discounted prior to the interpretation of such features as human in origin

grenvillian granulite facies metamorphism in the arequipa massif peru a laurentia gondwana link abstract the arequipa massif is a major component of the central andean orogenic basement extensively exposed along the coast of southern peru in the vicinity of the arica bight and bolivian orocline the southern part of the massif is dominated by granulitic gneisses for which a ca 1900 ma age for metamorphism of a slightly older protolith has been accepted on the basis of rb sr whole rock isochrons and bulk u pb zircon dating new u pb single grain zircon geochronology however reveals two mesoproterozoic granulite facies domains 1200 ma to the ne of quilca 16 44 s 72 24 w and 970 ma in the vicinity of mollendo 16 58 s 72 03 w these ages correspond to the major metamorphic events represented in the grenville province of north america and lend credence to proposed reconstructions based on sparse isotopic data that grenvillian metamorphic belts underlie the western margins of south american cratons and constituted the conjugate rifted margin of eastern laurentia the protolith ages of the samples further substantiates the proposed neoproteozoic reconstruction in both domains of the massif the orthogneisses have zircon upper intercept ages of about 1950 ma and the paragneisses contain younger detrital zircons suggesting sedimentation in the early mesoproterozoic correlative zircon ages are found in the makkovik ketilidian belt and the adjacent trans labrador batholith of eastern laurentia the arequipa massif is therefore envisaged to represent an exotic terrane that originated as the tip of a pre grenville laurentian promontory comprising labrador greenland and scotland that was embedded into the proto andean margin during the grenville orogeny

insights into the complexity of crustal differentiation k2o poor leucosomes within metasedimentary migmatites from the southern marginal zone of the limpopo belt south africa differentiation of the continental crust is the result of complex interactions between a large number of processes which govern partial melting of the deep crust magma formation and segregation and magma ascent to significantly higher crustal levels the anatectic metasedimentary rocks exposed in the southern marginal zone of the limpopo belt represent an unusually well exposed natural laboratory where the portion of these processes that operate in the deep crust can be directly investigated in the field the formation of these migmatites occurred via absent incongruent melting reactions involving biotite which produced cm to m scale k2o poor garnet bearing stromatic leucosomes with high ca na ratios relative to their source rocks field investigation combined with geochemical analyses and phase equilibrium modelling designed to investigate some aspects of disequilibrium partial melting show that the outcrop features and compositions of the leucosomes suggest several steps in their evolution 1 melting of a portion of the source with restricted plagioclase availability due to kinetic controls to produce a magma melt entrained peritectic minerals in variable proportions relative to melt 2 segregation of the magma at near peak metamorphic conditions into melt accumulation sites mas a k a future leucosome 3a reequilibration of the magma with a portion of the bounding mafic residuum via chemical diffusion h2o k2o which triggers the co precipitation of quartz and plagioclase in the mas 3b extraction of melt dominated magma to higher crustal levels leaving peritectic minerals entrained from the site of the melting reaction and the minerals precipitated in the mass to form the leucosome in the source the key mechanism controlling this behaviour is the kinetically induced restriction of the amount of plagioclase available to the melting reaction this results in elevated melt h2o and k2o and chemical potential gradient for these components across the leucosome mafic residuum contact the combination of all of these processes accurately explains the composition of the k2o poor leucosomes these findings have important implications for our understanding of melt segregation in the lower crust and minimum melt residency time which according to the chemical modelling is 5 years we demonstrate that in some migmatitic granulites the leucosomes constitute a type of felsic refractory residuum rather than evidence of failed magma extraction this provides a new insight into the ways that source heterogeneity may control anatexis this article is protected by copyright all rights reserved

the mesoproterozoic maz terrane in the western sierras pampeanas argentina equivalent to the arequipa antofalla block of southern peru implications for west gondwana margin evolution the rocks of sierra de maz and sierra del espinal western sierras pampeanas represent pre famatinian ordovician basement isotope compositions nd and pb of metasedimentary rocks and shrimp u pb dating of detrital zircons combined with other geological evidence show that three parallel n s domains can be recognized the central maz domain contains pre grenvillian metasedimentary rocks deposited between 1 2 and 1 6 ga that underwent grenvillian granulite facies metamorphism and were intruded by mafic igneous rocks and massif type anorthosites metasedimentary rocks have high nd tdm ages 1 7 2 7 ga and very radiogenic pb 9 8 10 2 suggesting provenance from reworked early proterozoic or archean continental crust the domains to the east and west of the maz domain consist of three metasedimentary sequences with nd tdm ages between 1 2 and 1 6 ga and variably radiogenic pb 9 6 10 0 u pb shrimp dating of detrital zircons nd tdm model ages and comparison with other data suggest that these sequences are post grenvillian i e neoproterozoic and or early paleozoic the maz domain is interpreted as a suspect terrane similar to the northern arequipa antofalla craton that forms the basement of the central andes both underwent grenville age orogeny and were probably once continuous along the western margin of amazonia west gondwana

spatial and temporal evolution of liassic to paleocene arc activity in southern peru unraveled by zircon u pb and hf in situ data on plutonic rocks cordilleran type batholiths are built by prolonged arc activity along active continental margins and provide detailed magmatic records of the subduction system evolution they complement the stratigraphic record from the associated forearcs and backarcs we performed in situ u pb geochronology and hf isotope measurements on zircon grains from a large set of plutonic rocks from the coastal batholith in southern peru this batholith emplaced into the precambrian basement and the mesozoic sedimentary cover we identify two major periods of voluminous arc activity during the jurassic 200 175 ma and the late cretaceous paleocene 90 60 ma jurassic arc magmatism mainly resulted in the emplacement of a dominantly mafic suite with ehf values ranging from 9 5 to 0 1 published ages south of the arequipa area suggest that the arc migrated southwestward out of the study area during the middle jurassic after a magmatic gap of 85 ma arc activity abruptly resumed 90 ma ago in arequipa intrusive bodies emplaced into both basement and older jurassic intrusions and strata this activity culminated between 70 and 60 ma with the emplacement of very large volumes of dominantly quartz dioritic magmas this last episode may be considered as a flare up event characterized by intense magmatic transfers into the crust and rapid relief creation the late cretaceous paleocene initial ehf are shifted toward positive values up to 3 3 and 2 6 compared to the jurassic ones indicating either a larger input of juvenile magmas a lesser interaction with the ancient crust or an increase of re melting of young mantle derived mafic lower crust these magmatic fluxes with juvenile component are coeval with the onset of the crustal thickening at 90 ma and represent a significant contribution to the formation of the continental crust in this area

ultra high temperature metamorphism and multistage decompressional evolution of sapphirine granulites from the palni hill ranges southern india sapphirine granulites from a new locality in the palni hill ranges southern india occur in a small enclave of migmatitic highly magnesian metapelites mg 85 72 within massive enderbitic orthogneiss they show a variety of multiphase reaction textures that partially overprint a coarse grained high pressure assemblage of bt opx ky grt pl qtz the sequence of reactions as deduced from the corona and symplectite assemblages together with petrogenetic grid considerations records a clockwise p t evolution with four distinct stages 1 equilibration of the initial high p assemblage in deep overthickened crust 12 kbar 800 900 c was followed by a stage of near isobaric heating presumably as a consequence of input of extra heat provided by the voluminous enderbitic intrusives during heating kyanite was converted to sillimanite and biotite was involved in a series of vapour phase absent melting reactions which resulted in the ultra high temperature assemblage opx crd kfs spr sil grt qtz bt coexisting with melt equilibration at c 950 1000 c 11 10 kbar 2 subsequently as a result of decompression of the order of 4 kbar at ultra high temperature a sequence of symplectite assemblages opx sil spr spr crdopx spr crdopx crdopx crd spl crd spl developed at the expense of garnet orthopyroxene and sillimanite this stage of near isothermal decompression implies rapid ascent of the granulites into mid crustal levels possibly due to extensional collapse and erosion of the overthickened crust 3 development of late biotite through back reaction of melt with residual garnet indicates a stage of near isobaric cooling to c 875 c at 7 8 kbar i e relaxation of the rapidly ascended crust to the stable geotherm 4 a second period of near isothermal exhumation up to c 6 5 kbar 850 c is indicated by the partial breakdown of late biotite through volatile phase absent melting reactions available isotope data suggest that the early part of the evolutionary history stages 1 3 is presumably coeval with the early proterozoic metamorphism in the extended granulite terrane of the nilgiri biligirirangan and shevaroy hills to the north while the exhumation of the granulites from mid crustal levels stage 4 occurred only during the pan african thermotectonic event which led to the accretion of the kerala khondalite belt to the south

ultrahigh temperature granulite metamorphism 1050 c 12 kbar and decompression in garnet mg70 orthopyroxene sillimanite gneisses from the rauer group east antarctica highly magnesian and aluminous migmatitic gneisses from mather peninsula in the rauer group eastern antarctica preserve ultrahigh temperature uht metamorphic assemblages that include orthopyroxene sillimanite quartz garnet sillimanite quartz and garnet orthopyroxene sillimanite garnet that ranges up to xmg of 71 5 coexists with aluminous orthopyroxene that shows zoning from cores with 7 5 8 5 wt al2o3 to rims with up to 10 6 wt al2o3 adjacent to garnet peak p t conditions of 1050 c and 12 kbar are retrieved from fe mg al thermobarometry involving garnet and orthopyroxene in very good agreement with independent constraints from petrogenetic grids in feo mgo al2o3 sio2 and related chemical systems sapphirine orthopyroxene and cordierite form extensive symplectites and coronas on the early phases the specific reaction textures and assemblages involving these secondary phases correlate with initial garnet xmg with apparent higher pressure reaction products occurring on the more magnesian garnet and are interpreted to result from an initial phase of ultrahigh temperature near isothermal decompression uht itd from 12 to 8 kbar at temperatures in excess of 950 c later textures that involved biotite formation and then partial breakdown along with garnet relics to symplectites of orthopyroxene cordierite or cordierite spinel may reflect hydration through back reaction with crystallizing melts on cooling below 900 850 c followed by itd from 7 to 8 kbar to c 5 kbar at temperatures of 750 850 c the tectonic significance of this p t history is ambiguous as the rauer group records the effects of archean tectonothermal events as well as high grade events at 1000 and 530 ma late stage biotite formation and subsequent itd can be correlated with the p t history preserved in the proterozoic components of the rauer group and hence with either 1000 or 530 ma collisional orogenesis however whether the preceding uht itd history reflects a temporally unrelated event e g archean or is simply an early stage of either the late proterozoic or pan african tectonism as recently deduced for similar uht rocks from other areas of the east antarctica remains uncertain

electron microprobe dating of monazite because monazite is extremely rich in u and th radiogenic pb pb accumulates very quickly and reaches in about 100 ma a level where it is possible to analyse it with the electron probe assuming that common pb is negligible and that partial loss of pb has not occurred the simultaneous measurement of u th and pb allows to obtain a geologically meaningful age from a single electron probe analysis here we present the results of two years of systematical investigations aiming to define both the limits and potential of this method a specific statistical method to deal with the large number of data which can be obtained on a single sample is described and several guidelines illustrated by examples are suggested to optimize the method electron probe measurements carried out on samples of known age from 200 ma to 3 1 ga yield ages that always fall inside the confidence interval of the isotopically determined age demonstrating that this method is reliable the younger age limit is approximately 100 ma although it can be younger in some favourable cases in old monazites extremely high pb contents have been found up to 5 wt indicating that monazite can tolerate high radiation doses without experiencing lead loss the final precision on the age for a normal monazite is 30 50 ma for a total counting time of 600 s a complete dating procedure can be completed in less than 1 h first results indicate that old ages can be preserved in monazite either in small relict cores in crystals or by the coexistence of several generations of monazites in a sample this method has all the advantages of the electron probe it is non destructive has an excellent spatial resolution monazites as small as 5 i m can be dated and because it is possible to work on normal polished thin sections the petrographical position of the dated crystal is known this method offers a large number of geologists access to an in situ dating technique at moderate cost

roof sagging limit in an early warning system for safe coal pillar extraction abstract field studies have found that generally a roof fall takes place after certain amount of roof strata sagging recent applications of auto warning tell tale awtt instrument as an early warning system at different depillaring panels found it to be simple economical and efficient during the coal pillar extraction called depillaring awtt is able to watch the set value of roof sagging continuously in time however the application of awtt experienced two practical issues i fixing roof sagging warning value before the fall of overlying strata and ii deciding horizon above which its anchorage point of awtt needs to be installed different field experiences noticed that an anchorage horizon at 10 m depth from the ceiling of a gallery works satisfactorily for goaf edge but fixation of the roof sagging warning value before the fall of overlying strata is found to be complex generally the warning value for a given site conditions is determined by extensive field monitoring and correlation of the roof sagging value at the goaf edge with corresponding roof falls field studies found that modulus of elasticity and thicknesses of immediate roof are important parameters in deciding the warning value of the roof sagging these studies also indicated that the variation in size of the left out ribs and goaf span influence the warning value these observed facts about the warning value for an awtt are studied in laboratory using a numerical modelling approach in flac3d the results obtained through developed simulated models are found to be in close agreement with those of the field studies attempts are also made to quantify the influence of above mentioned parameters over the warning value through collated results of different studied models the obtained results of field and laboratory investigations are presented and discussed in this paper for strata control in a depillaring panel

fluid absent melting of f rich phlogopite rutile quartz fluid absent melting is believed to be an important process in the generation of melts m the lower crust and upper mantle breakdown of phlogopite makes h 0 available and thus controls the conditions at which fluid absent melting occurs both f2andti in biotite ha e been shown to affect strongly the thermal stability of biotite to model better the fluid absenti eltmg of a ssemblagescontaining phlogopite the reaction f phlogopite quartz rutile enstatite melt has been studied experimentally experiments were pe o med at 7 10 and 15 kbar using a natural f rich phlogopite with a starting com os tion off f oh 0 43 and mg mg fe in molar proportions 0 94 results mdlcate that the therma l stability of f rich phlogopite quartz rutile is extended by as much as 450 c relative to the kmash system and by 300 c relative to the ti free f kmash system approach to equilibrium in the experiments was assessed by convergence of results of melting and crystallization experiments phlogopite compositions from e penmental products show that although f rich phlogopite incorporates relatively little ti 3 wt i02 the combination of f and ti increases the stability of phlogopite to considerably higher temperatures 300 0c than that of either component alone melts formed by the fluid absent melting off rich phlogopite quartz rutile at temperatures 1000 care anitic and stro g1yperaluminous the compositions of these melts suggest that the formationofmetalummous to peralkaline a type granites by fluid absent melting of halogen enriched sources is unlikely

theoretical phase relations involving cordierite and garnet revisited the influence of oxygen fugacity on the stability of sapphirine and spinel in the system mg fe al si o the theoreticalp t grid for stability relations of the phases cordierite cd sapphirine sa hypersthene hy garnet ga spinel sp sillimanite si and quartz qz of hensen 1971 has proved useful in the interpretation of metamorphic mineral assemblages formed at low oxygen fugacity both experimental data and evidence from natural rocks indicate that at high oxygen fugacity compatability relations change as a result of the enlargement of the stability field of spinel which causes a topological inversion and the stabilisation of the invariant points sa ga and cd this implies the stable existence of the univariant equilibria for f text o text 2 buffered conditions sp qz ga hy si o2 sa cd cd sp qz hy si o2 sa ga and sa sp qz hy si o2 ga cd and the divariant reaction sp qz hy si o2 sa ga cd these redox equilibria are restricted to conditions of high oxygen fugacity the proposed theoreticalp t grids for both low and high oxygen fugacity satisfactorily explain all experimental data and metamorphic mineral assemblages so far found in granulites

the spinel and quartz associations in high grade xenoliths from tallante s e spain and their potential use in geothermometry and barometry remarkable spinel quartz associations are observed in some xenoliths found in the basaltic rocks from tallante the main primary parageneses are quartz plagioclase sillimanite spinel garnet ilmenite graphite quartz plagioclase garnet orthopyroxene spinel ilmenite

upshot of strata movement during underground mining of a thick coal seam below hilly terrain underpinning based simultaneous extraction was successfully used to extract more than 90 of thick and contiguous sections of a coal seam at chirimiri mine lying below a hill cap with a rapid change in depth of cover and is placed above the surrounding ground level the last few panels of the mine encountered strata control problems during the depillaring underground instrumentation and observations showed that the last phase depillaring of the property near the free surface of the hill slope experienced shearing of overlying strata along two incompetent layers field investigations revealed that large amount of lateral movement due to the hilly nature of the overlying strata is the main source of the problem a simple two dimensional study on a simulated model also demonstrated the occurrence of the problem during working below an unconfined rock mass of hilly nature different evidences were collected from the site to identify the main horizons and the direction of lateral movement and accordingly two different strata management plans one for each section were successfully executed to overcome the encountered strata control problems field investigations in the rest of the panels of the site showed favourable impact of the adopted strata management plan

the amazon laurentian connection as viewed from the middle proterozoic rocks in the central andes western bolivia and northern chile middle proterozoic rocks underlying the andes in western bolivia western argentina and northern chile and early proterozoic rocks of the arequipa massif in southern peru form the arequipa antofalla craton these rocks are discontinuously exposed beneath mesozoic and cenozoic rocks but abundant crystalline clasts in tertiary sedimentary rocks in the western altiplano allow indirect samples of the craton near berenguela western bolivia the oligocene and miocene mauri formation contains boulders of granodiorite augen gneiss 1171 20 ma and 1158 12 ma u pb zircon quartzose gneiss and granofels that are inferred to have arkosic protoliths 1100 ma source region u pb zircon quartzofeldspathic and mafic orthogneisses that have amphibolite and granulite facies metamorphic mineral assemblages 1080 ma metamorphism u pb zircon and undeformed granitic rocks of phanerozoic age the middle proterozoic crystalline rocks from berenguela and elsewhere in western bolivia and from the middle proterozoic belen schist in northern chile generally have present day low 206pb 204pb 15 57 and elevated 208pb 204pb 37 2 to 50 7 indicative of high time averaged th u values the middle proterozoic rocks in general have higher present day 206pb 204pb values than those of the early proterozoic rocks of the arequipa massif 206pb 204pb between 16 1 and 17 1 but lower than rocks of the southern arequipa antofalla craton 206pb 204pb 18 5 a difference inferred to reflect grenvillian granulite metamorphism the pb isotopic compositions for the various proterozoic rocks lie on common pb isotopic growth curves implying that pb incorporated in rocks composing the arequipa antofalla craton was extracted from a similar evolving pb isotopic reservoir evidently the craton has been a coherent terrane since the middle proterozoic moreover the pb isotopic compositions for the arequipa antofalla craton overlap those of the amazon craton thereby supporting a link between these cratons and seemingly precluding part of the arequipa antofalla craton from being a detached fragment of another craton such as eastern laurentia which has been characterized by a different u pb history pb isotopic compositions for the arequipa antofalla craton are furthermore distinct from those of the proterozoic basement in the precordillera terrane western argentina indicating a pb isotopic and presumably a tectonic boundary between them the pb isotopic compositions for the precordillera basement are similar to those of eastern laurentia and support other data indicating that these rocks are a detached fragment of north america finally the distinct pb isotopic evolution history of the arequipa antofalla craton and eastern laurentia require minor modification to tectonic models linking eastern north america scotland to the oroclinal bend in western south america

cenozoic forearc basin sediments in southern peru 15 18 s stratigraphic and heavy mineral constraints for eocene to miocene evolution of the central andes abstract a large sedimentary forearc basin developed in cenozoic times between the present day coastal cordillera and the western cordillera of the central andes called moquegua basin in southern peru the basin is filled by moquegua group deposits 50 to 4 ma comprising mostly siliciclastic mudstones sandstones and conglomerates as well as volcanic intercalations several facies changes both along orogenic strike and through time are described and have led to subdivision into four sedimentary units moquegua a b c and d in this paper we present a refined stratigraphic scheme of the moquegua group combined with the first provenance analysis of the moquegua basin based on i semi quantitative analysis of heavy mineral abundance ii electron microprobe emp and laser ablation la icp ms analyses of single detrital amphibole and fe ti oxide grains and iii comparative analysis of the different potential source rocks to clearly identify the most likely sources results allow us to reconstruct sediment provenance and to relate changes of the erosion sedimentation system in the moquegua basin to the evolution of the andean orogen at 50 to 40 ma the moquegua basin was close to sea level and fed by low energy rivers transporting mainly metamorphic basement and jurassic cretaceous sedimentary detritus from local and distal sources the latter might be as far as the present eastern cordillera from 35 ma on the distal sediment sources were cut off by the uplift of the altiplano and eastern cordillera leading to higher energy fluvial systems and increasing importance of local sources especially the relevant volcanic arcs from 25 ma on volcanic arc rocks became the predominant sources for moquegua group sediments the 10 ma time lag observed between the onset of uplift induced facies and provenance changes at 35 ma and the onset of intense magmatic activity at 25 ma suggests that magmatic addition was not the main driver for crustal thickening and uplift in the central andes during latest eocene to oligocene time

multidisciplinary approach of the hyperarid desert of pampas de la joya in southern peru as a new mars like soil analog abstract the distribution of living organisms organic matter and chemical properties in mars like environments on earth can be used as a model to guide the investigation of possible habitable environments on mars this work aims to demonstrate that the place known informally as the mar de cuarzo sea of quartz in the pampas de la joya desert southern peru between 16 s and 17 s latitude contains soils with characteristics similar to those found on the martian surface using a multidisciplinary approach we studied the environmental data geology organic matter content oxidant activity and microbiology of this area our data show that 1 mar de cuarzo is a hyper arid area with a lower concentration of organic matter than those found in the mars like soils from yungay area atacama desert in chile while at the same time having comparable extreme environmental conditions and very low levels of microorganisms 2 the detrital components of the soils come essentially from the andean volcanic chain and local outcrops of precambrian gneisses and cretaceous granitic batholiths 3 the presence of microclimates geomorphological features and the high influence of the el nino southern oscillation enso allowed the formation of exotic and heterogeneous chemical deposits in these soils including iron oxides sulfates and other evaporites 4 thermal volatilization in these soils using methods similar to the viking and phoenix instruments shows high oxidant activity 5 labeled release experiment similar to the viking instrument shows high degradation of nutrients added in these soils altogether the mar de cuarzo area in the pampas de la joya is an interesting place for astrobiological studies as a new analog to mars and for comparative analyses with other hyperarid analogs as yungay

regional scale grenvillian age uht metamorphism in the mollendo camana block basement of the peruvian andes the mollendo camana block mcb is a 50 150 km precambrian inlier of the andean belt that outcrops along the pacific coast of southern peru it consists of stromatic migmatites of paleoproter ozoic heritage intensely metamorphosed during the grenville event c 1 ga u pb and u th pb ages on zircon and monazite in the migmatites aluminous mesosomes fmas and quartzofeldspathic leucosomes kfmash contain various amounts of k feldspar kfs orthopyroxene xmg opx 0 86 plagioclase pl sillimanite sil exceptionally kyanite ky ilmenite ilm magnetite mag quartz qtz and minor amounts of garnet xmg grt 0 60 sapphirine xmg spr 0 87 cordierite xmg crd 0 92 and biotite xmg bt 0 83 the ubiquitous peak mineral assemblage is opx sil kfs qtz grt in most of the mcb which together with the high al content of orthopyroxene 10 al2o3 and the local coexistence of sapphirine quartz attest to regional uht metamorphism 900 c at pressures in excess of 1 0 gpa fluid absent melting of biotite is responsible for the massive production of orthopyroxene that proceeded until exhaustion of biotite and most of the garnet in the southern part of the mcb mollendo cocachacra areas in this area a first stage of decompression from 1 1 1 2 to 0 8 0 9 gpa at temperatures in excess of 950 c is marked by the breakdown of sil opx to spr opx crd assemblages according to several bivariant fmas reactions high t decompression is also shown by mg rich garnet being replaced by crd spr and crd opx bearing symplectites and reacting with quartz to produce low al opx sil symplectites in quartz rich migmatites neither osumilite nor spinel quartz assemblages being formed isobaric cooling at about 0 9 gpa probably followed the initial decompression and proceeded with massive precipitation of melts towards the os invariant point as demonstrated by bt qtz pl symplectites in quartz rich migmatites melt opx sil bt grt kfs qtz finally opx rims around secondary biotite attest to late fluid absent melting compatible with a second stage of decompression below 900 c the two stages of decompression are interpreted as due to rapid tectonic denudation whereas the regional extent of uht metamorphism in the area probably results from large scale penetration of hot asthenospheric mantle at the base of an over thickened crust

the crustal evolution of south america from a zircon hf isotope perspective hf isotope data of 1100 detrital zircon grains from the palaeozoic south central andean gondwana margin record the complete crustal evolution of south america which was the predominant source the oldest grains with crustal residence ages of 3 8 4 0ga are consistent with complete recycling of existing continental crust around 4ga we confirm three major archaean palaeoproterozoic transamazonian and late mesoproterozoic to early neoproterozoic crust addition phases as well as six igneous phases during proterozoic to palaeozoic time involving mixing of juvenile and crustally reworked material a late mesoproterozoic to early neoproterozoic grenville age igneous belt can be postulated along the palaeo margin of south america this belt was the basement for later magmatic arcs and accreted allochthonous microcontinents as recorded by similar crustal residence ages crustal reworking likely dominated over juvenile addition during the palaeozoic era and proterozoic and archaean zircon was mainly crustally reworked from the eroding thickened ordovician famatinian arc

ultrahigh temperature metamorphism 1150 c 12 kbar and multistage evolution of mg al rich granulites from the central highland complex sri lanka mgand al rich granulites of the central highland complex sri lanka preserve a range of reaction textures indicative of a multistage p t history following an ultrahigh temperature metamorphic peak the granulites contain a near peak assemblage of sapphirine garnet orthopyroxene sillimanite quartz k feldspar which was later overprinted by intergrowth symplectite and corona textures involving orthopyroxene sapphirine cordierite and spinel biotite rims kornerupine and orthopyroxene rims on biotite are considered to be late assemblages thermobarometric calculations yield an estimated p t of at least 1100 c and 12 kbar for the near peak metamorphism isopleths of al2o3 in orthopyroxene are consistent with a peak temperature above 1150 c the p t path consists of four segments initial isobaric cooling after peak metamorphism segment a which produced the garnet sapphirine quartz assemblage was followed by near isothermal decompression at ultrahigh temperature segment b which produced the multiphase symplectites further isobaric cooling segment c resulted in the formation of biotite and kornerupine and late isothermal decompression segment d formed orthopyroxene rims on biotite this evolution can be correlated with similar p t paths elsewhere but there are not yet sufficient geochronological and structural data available from the highland complex to allow the tectonic implications to be fully assessed

cordierite as a monitor of fluid and melt h2o contents in the lower crust an experimental calibration cordierite can contain variable amounts of h 2 o and co 2 and the total amount and relative proportions of these volatiles reflect the environment in which the cordierite was formed in this study cordierite has been experimentally equilibrated with a peraluminous granite melt and the h 2 o contents of the phases have been determined by secondary ion mass spectrometry sims the results show that cordierite can have a range of h 2 o contents at a single pressure and temperature in this case 900 c 5 0 kbar the maximum h 2 o contents occur at saturation when h 2 o vapor is also present at which point cordierite contains 1 69 0 05 wt and the melt contains 10 0 0 42 wt however as the system becomes progressively more water undersaturated the phases contain less h 2 o and the partitioning coefficient d w h 2 o melt h 2 o cordierite in wt decreases from 5 9 0 3 to 4 4 0 6 the experiments show that equilibration with a volatile undersaturated melt provides an alternative to leakage as a mechanism for producing volatile poor cordierites application of this experimental calibration to cordierites from granulite migmatites predicts that the melts with which they last equilibrated contained only 2 7 3 8 wt h 2 o

sapphirine bearing fe rich granulites in the sw siberian craton angara kan block implications for paleoproterozoic ultrahigh temperature metamorphism abstract we report here for the first time the occurrence of sapphirine bearing rocks within the paleoproterozoic granulite complex of the angara kan block sw siberian craton and provide strong evidence for crustal metamorphism at ultrahigh temperature uht conditions in this region the uht rocks contain sapphirine high alumina orthopyroxene orthopyroxene sillimanite assemblage mesoperthitic feldspar and pseudomorphoses after osumilite crystals the peak temperature of metamorphism was estimated using al2o3 contents in opx up to 10 wt and the ternary feldspar composition and falls in a range of 1050 1100 c observed mineral microstructures indicate post peak cooling of rocks without a considerable decrease in pressure down to a temperature of about 900 c another type of rock related with uht metamorphism is represented by orthopyroxene sillimanite gneisses which recorded a peak temperature of about 950 c decreasing down to 900 c during post peak stage the uht gneisses record microstructural evidence of melt injection and matrix melt interaction near the peak metamorphic conditions sapphirine grains occur in the gneiss with relatively low mg bulk composition it is suggested that sapphirine occurs in the mg al rich domains which have been formed as a result of partial melting and precipitation of fe oxides from the melt high concentrations of al2o3 in orthopyroxene are the only sign of uht metamorphism in fe rich rock matrix which is not affected by melt new shrimp u pb zircon data from two uht gneisses indicate the presence of two well defined age peaks at ca 1 8 and 1 9 ga from rims and metamorphic grains and older dates from ca 1 91 to 3 15 ga from detrital cores the prominent feature of the metamorphic zircons is depletion of u with high th u ratio up to 6 0 steep hree patterns of the metamorphic zircons suggest their growth in the presence of a melt the metamorphic zircons from the uht gneiss record a broad range of temperatures between 1000 c and 810 c indicating protracted zircon growth from near peak conditions through retrograde stage of pt path the growth of the youngest zircons occurred at higher temperature indicating that the uht metamorphic conditions were reached at 1 78 1 8 ga the timespan of the granulite uht metamorphic event is near 50 ma and is limited by intrusions of late leucogranites and charnockites at 1 75 1 73 ga protracted interval of granulite uht metamorphism conforms with isobaric cooling p t path constructed by mineral assemblages

ground movement due to longwall mining in high relief areas in new south wales australia abstract the theoretical pattern of ground movement observed in flat terrains was modified by surface topography and geological structures in high relief areas e g creeks and gullies the observed pattern of movement was asymmetrical about the centre of the extraction panel the lateral movement of both sides of the valley caused large compressive strains and a hump at the creek bed the strains were three four times those normally observed in flat or fairly gentle topographies the hump was due to the rock mass in the creek bed being pushed up by the lateral forces from the valley sides a major geological fault caused both sides of the valley to move laterally away from the fault plane the magnitude of the maximum horizontal movement was as large as 40 of the observed vertical movement outside the goaf the horizontal movements were generally larger than the corresponding vertical movements however the horizontal movements outside the goaf appeared to be rigid body type movements with small deformations which may only have insignificant surface consequences

the impact of key strata movement on ground pressure behaviour in the datong coalfield abstract in situ investigations are the most direct and authentic approach for determining the law of strata movement in this study in situ investigation was carried out to determine the law of fracture and movement of a key stratum four point surface extensometer fpse inside overlying strata and borehole camera observation were used in the overlying strata of a fully mechanized caving face of a super thick coal seam in the datong coalfield the time space correspondence between the fracture and movement of the key stratum and ground pressure in stopes was established in combination with observations of the ground pressure of the working face results show that the primary key stratum of the overlying strata is the main controlling stratum and its fracture and movement are the underlying cause of strong ground pressure behaviours such as rib spalling roof fall and support crushing the findings suggest that the primary key stratum can be weakened and the energy release can be decreased by hydraulic fracturing deep hole blasting and other measures thus relieving ground pressure behaviour the research method of establishing the time space correspondence between fracture and movement of the key stratum and ground pressure behaviours based on in situ investigation can provide guidance for mining engineering schemes such as exploring master control strata with strong ground pressure behaviour under similar conditions and determining a reasonable capacity for the support

the distribution of h2o co2 between cordierite and granitic melt under fluid saturated conditions at 5 kbar and 900 c experiments to determine the distribution of h2o and co2 between synthetic peraluminous granitic melt and natural fe mg cordierite have been conducted at 5 kbar and 900 c under fluid saturated conditions up to seven charges saturated with a range of h2o co2 fluid compositions were run simultaneously the h2o and co2 contents of the cordierites and the h2o contents of the melts were determined using secondary ion mass spectrometry as the co2 content of the fluid increases the h2o content of both the cordierite and the melt decrease in a similar manner to fluid undersaturated experiments in the h2o only system the cordierite h2o contents in the experiments range from 0 5 to 1 7 wt h2o the distribution coefficients dw wt h2o melt wt h2o cordierite are in the range 3 5 5 9 and agree with the model of harley and carrington for 5 kbar and 900 c the co2 content of the cordierite increases from 0 to 1 wt co2 in the most co2 rich experiment as the cordierite and melt h2o contents decrease our h2o co2 cordierite data allow us to model and predict the maximum co2 content in cordierite to be 0 16 0 01 molecules per formula unit i e 1 16 wt co2 at 5 kbar and 900 c this value and the channel co2 contents of cordierite in equilibrium with h2o co2 fluids at high xco2 are significantly lower than predicted using previous models on the basis of these experiments cordierites that are saturated in h2o co2 and have channel xco2 0 25 require aco2 of over 0 75

history of crustal growth and recycling at the pacific convergent margin of south america at latitudes 29 36 s revealed by a u pb and lu hf isotope study of detrital zircon from late paleozoic accretionary systems detrital zircon provides a powerful archive of continental growth and recycling processes we have tested this by a combined laser ablation icp ms u pb and lu hf analysis of homogeneous growth domains in detrital zircon from late paleozoic coastal accretionary systems in central chile and the collisional guarguaraz complex in w argentina because detritus from a large part of w gondwana is present here the data delineate the crustal evolution of southern south america at its paleopacific margin consistent with known data in the source regions zircon in the guarguaraz complex mainly displays an u pb age cluster at 0 93 1 46 ga similar to zircon in sediments of the adjacent allochthonous cuyania terrane by contrast zircon from the coastal accretionary systems shows a mixed provenance age clusters at 363 722 ma are typical for zircon grown during the braziliano pampean famatinian and post famatinian orogenic episodes east of cuyania an age spectrum at 1 00 1 39 ga is interpreted as a mixture of zircon from cuyania and several sources further east minor age clusters between 1 46 and 3 20 ga suggest recycling of material from cratons within w gondwana the youngest age cluster 294 346 ma in the coastal accretionary prisms reflects a so far unknown local magmatic event also represented by rhyolite and leucogranite pebbles it sets time marks for the accretion history maximum depositional ages of most accreted metasediments are middle to upper carboniferous a change of the accretion mode occurred before 308 ma when also a concomitant retrowedge basin formed initial hf isotope compositions reveal at least three juvenile crust forming periods in southern south america characterised by three major periods of juvenile magma production at 2 7 3 4 ga 1 9 2 3 ga and 0 8 1 5 ga the 176hf 177hf of mesoproterozoic zircon from the coastal accretionary systems is consistent with extensive crustal recycling and addition of some juvenile mantle derived magma while that of zircon from the guarguaraz complex has a largely juvenile crustal signature zircon with pampean famatinian and braziliano ages 660 ma originated from recycled crust of variable age which is however mainly mesoproterozoic by contrast the carboniferous magmatic event shows less variable and more radiogenic 176hf 177hf pointing to a mean early neoproterozoic crustal residence this zircon is unlikely to have crystallized from melts of metasediments of the accretionary systems but probably derived from a more juvenile crust in their backstop system

ground subsidence observations and a modified influence function method for complete subsidence prediction abstract the results of surface subsidence investigations carried out in different coal fields in india are presented in this paper important differences are seen to exist between the subsidence norms given in the subsidence engineers handbook uk and those obtained in indian conditions differences also exist for extraction below unfractured or uncaved rock and below pre existing caved goaf s the subsidence is higher in the latter case and also in the slopes and strains consequently separate equations and guidelines have been given for these two conditions of mining since indian coal mining is mostly under shallow to moderate cover several cases of discontinuous subsidence have been observed a demarcation limit for discontinuous subsidence is given seeing that discontinuous subsidence causes severe surface damage since room and pillar mining is the principal underground method of extraction panels of irregular shapes often occur making complete subsidence prediction difficult the influence function method with a new function and additional functions for modelling subsidence asymmetry and the effect of extraction edges are given in this paper the method for extraction below goaf s is also examined four cases are given for the comparison of observed and predicted subsidence contours and two cases for comparison of the slopes and strains

dynamic loading induced by the instability of voussoir beam structure during mining below the slope abstract dynamic loading events in the longwall faces under slopes in the shendong coalfield china seriously limit the safety and efficiency of mining the slope characteristics such as direction height and angle determine the load type and the stability of the voussoir beam structure vbs formed by fractured blocks of the primary key stratum pks whether or not dynamic loading events will occur can be directly determined based on the presence or absence of a stable vbs in the pks based on this relationship six structural mechanical vbs models for different load distributions were established and then the effects of the slope direction height and angle on the stability of vbs was analysed changes of load type intensified the compression force between the hinged blocks when the compression force between blocks exceeded the strength of the rock vbs instability and dynamic loading events occurred the compression force in the vbs increased with increasing slope angle and height when the slope directions were similar becoming more pronounced for mining under the uphill sections of the slopes therefore a method of determining the critical slope parameter values indicating a risk of dynamic loading was proposed enabling such slopes to be recognised in advance measures such as slope top cutting or backfilling can then be applied to prevent potential dynamic loading accidents this study can provide guidelines for safe mining under geological conditions involving widespread slopes and gullies

the effects of the rotational speed of voussoir beam structures formed by key strata on the ground pressure of stopes abstract underground coal mining leads to fracture and movement in the overburden and the occurrence of weighting events in stopes there are certain key strata that play an important role in controlling overburden movements after the key stratum breaking the fractured blocks would form the voussoir beam structure the rotational speed of the voussoir beam structures directly affects the intensity of the ground pressures in a stope in this study a mechanical model was constructed to describe the rotational motions of the voussoir beam structures formed by the key strata based on elastomechanical theories the angular velocity equations for the rotational motions of the voussoir beam structures in the key strata were obtained moreover the manner in which loads and block lengths affect the angular velocity was investigated the rotational angular velocity of the fractured blocks was found to increase with greater overlying loads on the key strata and shorter block lengths the main roof has a major influence on strata behavior in stopes and increases in the angular velocity of its voussoir beam structures increase roof subsidence rates the yield of powered supports within each coal mining cycle and the intensity of weighting events in stopes if one or more of the voussoir beam structures formed by key strata above the main roof have greater rotational speeds than those of their underlying key strata additional loads are applied to the latter causing their rotational speeds to increase if this application of loads is transmitted to the main roof there is a strong possibility that highly intense weighting events will occur as a result of increases in the rotational speed of the main roof s voussoir beam structures the research result provided a rational explanation for the significant differences in the weighting events of the no 52304 working face in the daliuta coal mine which was covered by thick versus thin aeolian sands the results may be greatly significant in evaluating the intensity of weighting events predicting the scope in which the strata movement affects the weighting events in stopes and determining a reasonable powered support capacity

context and environment in taphonomic analysis examples from alaska s porcupine river caves abstract field investigations of caves along alaska s porcupine river document three major mechanisms which modify bone in patterns similar to alterations produced by man 1 carnivore fracture 2 rodent gnawing and 3 rock fall and rubble scarring a late wisconsin faunal assemblage composed of equus sp rangifer tarandus ovis dalli bison sp proboscidean numerous small mammal species birds and fish is well documented this faunal assemblage suggests a mosaic environment of grassland tundra forest in the immediate vicinity of these caves and implies that the late wisconsin environment in north central alaska may have been characterized by a number of microenvironments and colder dryer steppe conditions taphonomic data which have historically been interpreted to support human occupation of eastern beringia during the pleistocene are critically examined and the context of these discoveries not the specimens themselves provides the test essential to document the antiquity of man in north america prior to 12 000 yr ago

two stage decompression in orthopyroxene sillimanite granulites from forefinger point enderby land antarctica implications for the evolution of the archaean napier complex magnesian metapelites of probable archaean age from forefinger point sw enderby land east antarctica contain very high temperature granulite facies mineral assemblages which include orthopyroxene 8 9 5 wt al2o3 sillimanite garnet quartz k feldspar that formed at 10 1 5 kbar and 950 50 c these assemblages are overprinted by symplectite and corona reaction textures involving sapphirine orthopyroxene 6 7 wt al2o3 cordierite and sometimes spinel at the expense of porphyroblastic garnet or earlier orthopyroxene sillimanite these textures mainly pre date the development of coarse biotite at the expense of initial mesoperthite and the subsequent formation of orthopyroxene 4 6 wt al2o3 cordierite plagioclase rinds on late biotite the early reaction textures indicate a period of near isothermal decompression at temperatures above 900 c decompression from 10 1 5 kbar to 7 8 kbar was succeeded by biotite formation at significantly lower temperatures 800 850 c and further decompression to 4 5 1 kbar at 700 800 c the later parts of this p t evolution can be ascribed to the overprinting and reworking of the forefinger point granulites by the late proterozoic c 1000 ma rayner complex metamorphism but the age and timing of the early high temperature decompression is not known it is speculated that this initial decompression is of archaean age and therefore records thinning of the crust of the napier complex following crustal thickening by tectonic or magmatic mechanisms and preceding the generally wellpreserved post deformational near isobaric cooling history of this terrain