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

late quaternary slip rates for the southern elsinore fault in the coyote mountains southern california from analysis of alluvial fan landforms and clast provenance soils and u series ages of pedogenic carbonate abstract offset alluvial fans along the elsinore fault in the south central coyote mountains were studied to resolve an average late quaternary slip rate for this major western strand of the san andreas fault system in southern california alluvial fans and their offsets were mapped using high resolution dems combined with field observations of fan surface morphology and the character of the soils developed in each fan remnant clast assemblage data was used to determine the source of each alluvial fan upstream of the fault and u series dating of pedogenic carbonate was used to estimate minimum ages of the alluvial fan surfaces forty u th dates on pedogenic carbonate confirm the utility of the technique for dating late pleistocene alluvium in arid regions and suggest that age variation among late pleistocene fans grouped on the basis of soils and geomorphic criteria may be significant based on these data the southernmost segment of the elsinore fault has sustained a slip rate of 2 4 0 4 mm y for the past 60 70 ka and probably for the past 150 ka because displacement in the most recent surface rupture increases northwest of our slip rate sites this rate is likely a minimum for the southern elsinore fault with the actual rate more likely close to 3 mm y in the central part of the range these new data confirm that slip gradients along individual fault segments must be considered when estimating pre holocene slip rates for seismic hazard estimates these new results show that the southern elsinore fault accounts for about 6 of the total relative motion between north america and the pacific lithospheric plates in southernmost california assessment of previous estimates of slip in the most recent event suggests earthquakes of about mw 6 8 and when combined with the slip rate data a recurrence of such events about every thousand years

sm nd evidence for diachronous crustal accretion in the lewisian complex of northwest scotland sm nd whole rock analyses of several cogenetic rock suites from the late archaean lewisian complex of northwest scotland yield chronological data for a crustal accretion differentiation superevent cads in this terrane three layered mafic ultramafic bodies predate badcallian high grade metamorphism 2660 ma but their temporal relationship to the igneous precursors of the host quartzofeldspathic gneiss complex remains unclear regression of data from these samples yield the following achiltibuie 2850 95 ma with nd t 1 93 0 52 drumbeg 2910 55 ma with nd t 1 70 0 32 scouriemore 2670 110 ma with nd t 0 98 0 73 these data suggest that contemporaneous sub lewisian mantle had a long term light rare earth element depleted history relative to a chondritic reservoir composition three suites of quartzofeldspathic gneisses the pyroxene granulites of scourie the hornblende granulites of gruinard bay and the amphibolites of the northern region north of loch laxford have small ranges of sm nd ratios and do not yield precise regression results if these gneisses were derived from a relatively homogeneous source region perhaps represented by the mafic ultramafic bodies then sm nd model ages suggest a diachronous sequence of crustal generation decreasing in age from scourie terrane average tdm 2930 ma through gruinard bay 2860 ma to the northern region 2780 ma a diachronous sequence such as this suggests that for a cads in which separate accretion and differentiation episodes may be resolved final high grade metamorphism may not be related to magmatic crustal generation processes

lewisian crustal history from in situ sims mineral chronometry and related metamorphic textures abstract a combination of detailed petrography and in situ sims 207 pb 206 pb micro chronometry of monazite has been carried out on samples from the lewisian high grade metamorphic terrain northwest scotland two episodes of granulite facies metamorphism have been recognized the thermal maxima of which are estimated at or before 2760 12 ma 2 and at 2526 8 ma respectively pegmatites at scourie crystallised at 2480 ma the amphibolite facies metamorphism of the main laxfordian event is estimated at 1752 12 ma and the formation of kyanite and staurolite is associated with this event the oldest ages of 2760 ma have been obtained from micro inclusions of monazite in garnet it appears that garnet which has a high closure temperature for the u pb system plays an important role in the behaviour of the monazite u pb system it serves to shield monazite from reaction with the other phases and to armour monazine from diffusive pb loss or from resetting

aluminous granulites of the archean craton of kasai zaire petrology and p t conditions aluminous granulites of the archean 2 8 ga kasai craton zaire consist of two main mineral assemblages grt opx and sil grt crd rocks the high grade metamorphic conditions as deduced from grt opx and grt opx pl qtz equilibria are 720 c 6 7 kb considering the zoning of the same minerals the slope of the p t path is estimated at 15 b c thermobarometry involving crd is consistent with those p t conditions three cordierite forming reactions have been observed petrographically grt qtz crd opx r1 grt sil crd spl r2 grt sil crd mag r3 these equilibria are continuous reactions end member reactions have slopes less than 15 b c they are decompression reactions occurring after the metamorphic climax using available thermodynamic data r3 fixes the oxygen fugacity to a value below the qfm buffer log 10f o2 17 6 at 720 c 6 7 kb and xh2ofl 0 3 in the graphite stability field the absence of graphite in the rocks shows that the end of the granulite facies metamorphism did not occur under important co 2 streaming the polymetamorphic history of this archean craton is considered 1988 oxford university press

a sm nd isotopic study of the south harris igneous complex the outer hebrides sm nd geochronology may be used to bracket the age of metamorphism in rocks which are difficult to date by other methods by coupling whole rock sm nd analyses of the principal members of the south harris igneous complex with sm nd mineral isochrons on two anothositic gabbros the age of granulite facios metamorphism has been defined whole rock analyses of three pairs of closely spaced samples of the anorthosite give consistent ages averaging 2 18 0 06 gyr but in general the data from the anorthosite do not define an isochron as a result of variable contamination of the evolving magma chamber whole rock data on the tonalite indicate that it is younger than 2 06 gyr its mean tchur age is 1 86 0 05 gyr garnet pyroxene amphibole plagioclase mineral isochrons on two anorthosite samples give identical 1 87 0 04 gyr ages which date cooling after the high pressure granulite facies metamorphism together with the tonalite whole rock data this defines the age of that metamorphism and confirms dearnley s original assignment of an early laxfordian age

sm nd dating and cooling history of scourian granulites sutherland the scourian gneisses of the lewisian complex of north west scotland have been the subject of many geochronological investigations sm nd whole rock measurements suggest that the protoliths of the lewisian complex differentiated 2 92 0 05 gyr ago from an approximately chrondritic mantle1 rb sr and pb pb whole rock ages between 2 8 and 2 6 gyr reflect subsequent depletion in rb and u refs 2 3 which has generally been associated with granulite facies metamorphism zircons which are thought to have crystallized during this metamorphism have also given u pb ages of 2 66 gyr ref 4 this granulite facies metamorphism involving pressures in excess of 10 kbar and peak temperatures estimated between 820 50 c ref 5 and 1 250 c refs 6 7 affects a terrain including supracrustal rocks7 and thus implies a major tectonic event we report here the first geochronological data measured on mineral assemblages that have also provided estimates of the conditions of metamorphism such measurements are possible because the major minerals of granulite facies assemblages show an unusually favourable spread in sm nd ratios garnet especially is strongly enriched in sm ref 8 data for three garnetiferous basic gneiss assemblages suggest that closure of the sm nd system occurred significantly later than the peak of the metamorphism

geomorphic controls on biological soil crust distribution a conceptual model from the mojave desert usa abstract biological soil crusts bscs are bio sedimentary features that play critical geomorphic and ecological roles in arid environments extensive mapping surface characterization gis overlays and statistical analyses explored relationships among bscs geomorphology and soil characteristics in a portion of the mojave desert usa these results were used to develop a conceptual model that explains the spatial distribution of bscs in this model geologic and geomorphic processes control the ratio of fine sand to rocks which constrains the development of three surface cover types and biogeomorphic feedbacks across intermontane basins 1 cyanobacteria crusts grow where abundant fine sand and negligible rocks form saltating sand sheets cyanobacteria facilitate moderate sand sheet activity that reduces growth potential of mosses and lichens 2 extensive tall moss lichen pinnacled crusts are favored on early to late holocene surfaces composed of mixed rock and fine sand moss lichen crusts induce a dust capture feedback mechanism that promotes further crust propagation and forms biologically mediated vesicular av horizons the presence of thick biogenic vesicular horizons supports the interpretation that bscs are long lived surface features 3 low to moderate density moss lichen crusts grow on early holocene and older geomorphic surfaces that display high rock cover and negligible surficial fine sand desert pavement processes and abiotic vesicular horizon formation dominate these surfaces and minimize bioturbation potential the biogeomorphic interactions that sustain these three surface cover trajectories support unique biological communities and soil conditions thereby sustaining ecological stability the proposed conceptual model helps predict bsc distribution within intermontane basins to identify biologically sensitive areas set reference conditions for ecological restoration and potentially enhance arid landscape models as scientists address impacts of climate change and anthropogenic disturbances

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

archaean crustal development in the lewisian complex of northwest scotland the lewisian complex of northwest scotland is typical of many archaean terrains and has a well documented history starting 2 700 myr ago1 6 here we present new isotopic data that extend this history back to 3 300 myr and provide some insight into how the earliest continental crust may have formed the lewisian is dominated by tonalite trondhjemite and granodiorite ttg rocks which require a mafic lithospheric source rather than being direct mantle melts7 11 but the mafic and ultramafic rocks in the high grade granulite facies part of this terrain show little evidence of a significantly older crustal history12 13 and precursor material to the ttg lithologies has not yet been identified here we show that older amphibolite material has survived at a lower metamorphic grade coexisting amphibolite minerals yield indistinguishable 207pb 206pb and 147sm 143nd ages of 3 310 27 myr and 3 298 73 myr respectively these data are consistent with an origin for much of the lewisian terrain by the re melting of preexisting lithosphere with an isotopic signature similar to that of the amphibolites studied here

proterozoic anticlockwise p t path of the lewisian complex of south harris outer hebrides nw scotland the leverburgh belt and south harris igneous complex in south harris northwest scotland experienced high pressure granulite facies metamorphism during the palaeoproterozoic the metamorphic history has been determined from the following mineral textures and compositions observed in samples of pelitic quartzofeldspathic and mafic gneisses especially in pelitic gneisses from the leverburgh belt 1 some coarse grained garnet in the pelitic gneiss includes biotite and quartz in the inner core sillimanite in the outer core and is overgrown by kyanite at the rims 2 garnet in the pelitic gneiss shows a progressive increase in grossular content from outer core to rims 3 the alvi aliv ratio of clinopyroxene from mafic gneiss increases from core to rim 4 retrograde reaction coronas of cordierite and hercynite cordierite are formed between garnet and kyanite and orthopyroxene cordierite and orthopyroxene plagioclase reaction coronas develop between garnet and quartz 5 a p t path is deduced from inclusion assemblages in garnet and from staurolite breakdown reactions to produce garnet sillimanite and garnet sillimanite hercynite with increasing temperature and 6 in sheared and foliated rocks hydrous minerals such as biotite muscovite and hornblende form a foliation modifying pre existing textures the inferred metamorphic history of the leverburgh belt is divided into four stages as follows m1 prograde metamorphism with increasing temperature m2 prograde metamorphism with increasing pressure m3 retrograde decompressional metamorphism with decreasing pressure and temperature and m4 retrograde metamorphism accompanied by shearing peak p t conditions of the m2 stage are 800 30 c 13 14 kbar pressure increasing from m1 to m2 suggests thrusting of continental crust over the south harris belt during continent continent collision the inferred p t path and tectonic history of the south harris belt are different from those of the lewisian of the mainland

the stability of sapphirine quartz and hypersthene sillimanite quartz assemblages an experimental investigation in the system feo mgo al 2 o 3 sio 2 under h 2 o and co 2 conditions phase relations and mineral chemistry involving the phases garnet gt spinel sp hypersthene hy sapphirine sa cordierite cd sillimanite sil and quartz qz have been experimentally determined in the system fmas feo mgo al2o2 sio2 under low fo2 and for various h2o co2 conditions several compositions were studied with 100 mg mg fe ratio ranging from 64 to 87 with excess quartz and sillimanite our data do not show any differences in gt cd stability and composition as a function of h2o co2 and h2o co2 ch4 content in good agreement with a previous experimental study at lower temperature aranovich and podlesskii 1983 at 1 000 c and 11 kbar under co2 saturated conditions cordierite grew from a crystalline mix unseeded with cordierite thus under water absent conditions cordierite will have a high p stability field in the presence of co2 if water has a pressure stabilizing effect on cordierite then our results would indicate that the effects of h2o and co2 are of the same magnitude at high temperature our data support the theoretical p t grid proposed by hensen 1986 for high t metapelites and are largely consistent with the high temperature experimental data of hensen and green 1973 the univariant boundary gt cd hy sil qz which marks the disappearance of hy sil qz assemblages has a negative dp dt slope above 1 000 c and a positive one below this temperature extrapolation of our data to iron free systems shows that the high p breakdown limit of mg cordierite has a negative slope in the range 1 025 1 300 c and probably positive below 1 000 c this indicates a maximum of stability for mg cordierite at around 1 000 c and 13 kbar because of the curvature of the univariant reactions en sil py qz mg cd en sil qz and gt cd hy sil qz the iron free invariant point involving the phases py en cd sil and qz probably does not exist sapphirine qz bearing assemblages are stable only at temperatures above 1 050 c at 1 075 c the joint gt sa is stable up to 11 kbar at higher pressure garnet sapphirine and quartz react according to the reaction gt sa qz hy sil reequilibrated sapphirines are more aluminous than the theoretical endmember mg2al4sio10 due to alal mgsi substitutions 100 al2o3 al2o3 feo mgo in experimental sapphirines ranges from 50 5 to 52 2 sapphirine in the assemblage sa cd sil qz shows a decrease in al content with decreasing temperature and pressure such that the alumina isopleths for sapphirine have a slight negative dp dt slope a similar decrease in al content of sapphirine with temperature is also observed in sa sil qz assemblages

polymetamorphic evolution of the lewisian complex nw scotland as recorded by u pb isotopic compositions of zircon titanite and rutile u pb isotopic relations in zircon and titanite of granulite and amphibolite gneisses in the lewisian complex and bordering laxford front reveal complex discordance patterns indicating multiple late archean and early proterozoic crystallization overgrowth and pb loss events the earliest stages in the evolution of the complex remain poorly resolved zircon ages of 2710 ma date high grade metamorphism and magmatism probably related to tectonic and magmatic accretion in a continental arc setting a distinct event at 2490 2480 ma possibly initiated by metamorphism and deformation at high grade conditions caused amphibolitization of the granulites and emplacement of granitic pegmatites this event can be correlated with development of inverian shear zones and formation of granitoid layers along the laxford front the emplacement of a younger generation of granitoid sheets during the laxfordian event fromed hydrothermal titanite at 1754 ma in gneisses south of the laxford bridge and partially reset older titanite at scourie growth of secondary titanite and rutile also occurred during subsequent low grade metamorphism at 1690 1670 ma

new evidence for protolith ages of lewisian granulites northwest scotland the granulite facies scourian rocks in the central region of the lewisian complex of northwest scotland play an important role in the understanding of the development of mid archean high grade gneiss complexes whereas the later largely proterozoic history of the scourian has been clarified in a detailed u pb study unraveling the archean ages of the protoliths using conventional isotope techniques has proved impossible in order to advance our understanding of the deep crustal processes responsible for these granulites it is essential to establish the age s of accretion of the protoliths and their subsequent tectonothermal history a combined cathodoluminescence cl and sensitive high mass resolution ion microprobe shrimp single zircon study has revealed a hitherto unrecognized morphological complexity in zircons from the type localities of the granulites use of cl allowed identification of relict oscillatory igneous zoning metamorphic overgrowths and irregular areas of recrystallization from the shrimp data an age of ca 2960 ma is inferred for the gneiss protoliths which were altered considerably during an important metamorphic event ca 2490 ma

p t grids for silica undersaturated granulites in the systems mas n 4 and fmas n 3 tools for the derivation of p t paths of metamorphism abstract considering the minerals cordierite cd sapphirine sa hypersthene hy garnet ga spinel sp sillimanite si and corundum co in the system feo mgo al2o3 sio2 fmas the stable invariant points are co ga cd and sa constraints imposed by experimental data for the system mas indicate that under low ph2o conditions the invariant points occur at high temperature 900 c and intermediate pressure 7 10 kbar this temperature is higher than that commonly advocated for granulite facies metamorphism in granulites fe mg exchange geothermometers may yield temperatures of 100 150 c below peak metamorphic conditions and evidence for peak temperatures is best preserved by relict high temperature assemblages and by al rich cores in orthopyroxene application of the fmas grid to some well documented granulite occurrences introduces important constraints on their p t histories rocks of different bulk compositions occurring in close proximity in the field may record distinct segments of their p t paths this applies particularly to rocks with evidence for reaction in the form of coronas symplectites and zoned minerals consideration of curved reaction boundaries and xms isopleths may explain apparently contradictory results for the stability of cordierite obtained from low temperature experiments and thermochemical calculations on the one hand and hightemperature experimental data on the other

u pb isotopic evidence for the accretion of different crustal blocks to form the lewisian complex of northwest scotland single zircon and titanite u pb shrimp data presented for tonalite trondhjemite granodiorite ttg suite gneisses and an ultramafic rock from the northern and central regions of the lewisian complex of northwest scotland show that protolith ages of tonalitic gneisses in the northern region 2800 2840 ma are significantly younger than those in the central region 2960 3030 ma further evidence of a major 2490 2480 ma metamorphic event in the central region is documented by a metamorphic zircon associated with a granulite facies ultramafic body a dioritic gneiss from the northern region has also been dated at c 2680 ma the northern region therefore does not comprise reworked central region rocks and consequently the old models for the evolution of the lewisian which were based upon this concept need replacing it is instead proposed that two distinct crustal blocks now the northern and central regions were tectonically juxtaposed along a boundary corresponding to the laxford front juxtaposition would appear to have occurred in proterozoic times as it must have postdated the 2490 2480 ma inverian metamorphism recorded only in the central region and the emplacement of granite sheets restricted to the northern side of the boundary the first recorded event common to both regions is resetting of titanite ages associated with c 1750 ma laxfordian amphibolite facies metamorphism zircon inheritance in rocks of both regions is scarce within one zircon from the northern region a c 3550 ma core was found this represents the oldest known material from the region

metamorphic evolution of aluminous granulites from labwor hills uganda sapphirine cordierite quartz and spinel cordierite quartz form relic assemblages of probable archaean age in fe rich aluminous metapelites from labwor hills uganda and reflect an unusually high temperature metamorphism 1 000 c at pressures in the vicinity of 7 9 kbars and a o2 near the magnetite hematite buffer subsequent reaction textures include the replacement of spinel and cordierite by sillimanite and hypersthene and formation of sapphirine hypersthene k feldspar quartz symplectites which are interpreted as pseudomorphs after osumilite a petrogenetic grid appropriate to these assemblages suggests these reaction textures may be due to cooling at constant or increasing pressure and constant a o2 or decreasing a o2 at constant temperature and pressure the former interpretation is supported by the coexistence of ilmenohematite and magnetite during the development of the reaction textures and by the comparatively low al2o3 contents of secondary hypersthene this pressure temperature path implies that 1 metamorphism occurred at deep levels within normal thickness crust probably less than 40 45 km thick due to an extreme thermal perturbation induced either by emplacement of mantle derived magmas or by thinning of the subcontinental lithosphere in an extensional tectonic regime 2 the excavation and surface exposure of the granulites is due to a subsequent postgranulite facies metamorphism crustal thickening most probably involving their incorporation into an allochthonous upper crustal thrust sheet during the formation of the mozambique foldbelt

reaction textures in a suite of spinel granulites from the eastern ghats belt india evidence for polymetamorphism a partial petrogenetic grid in the system kfmash and the roles of zno and fe2o3 spinel granulites with or without sapphirine occur as lenses in garnetiferous quartzofeldspathic gneisses leptynites near gokavaram in the eastern ghats belt india spinel granulites are mineralogically heterogeneous and six mineral associations occur in closely spaced domains these are i spinel quartz cordierite ii spinel quartz cordierite garnet orthopyroxene sillimanite iii spinel cordierite orthopyroxene sillimanite iv spinel quartz sapphirine sillimanite garnet v spinel quartz sapphirine garnet and iv rhombohedral fe ti oxide cordierite orthopyroxene sillimanite common to all the associations are a porphyroblastic garnet containing an internal schistosify defined by biotite sillimanite and quartz perthite and plagioclase spinel contains variable amounts of exsolved magnetite and is distinctly zn rich in the sapphirine absent associations x mg in the coexisting phases decreases in the order cordierite biotite sapphirine orthopyroxene spinel garnet fe ti oxides textural criteria and compositional characteristics of the phases document several retrograde mineral reactions which occurred subsequent to prograde dehydration melting reactions involving biotite sillimanite quartz plagioclase and spinel the following retrograde mineral reactions are deduced 1 spinel quartz cordierite 2 spinel quartz garnet sillimanite 3 garnet quartz cordierite orthopyroxene 4 garnet quartz sillimanite cordierite 5 spinel cordierite orthopyroxene sillimanite 6 spinel sillimanite quartz sapphirine 7 spinel sapphirine quartz garnet sillimanite and 8 spinel quartz sapphirine garnet a partial petrogenetic grid for the system feo mgo al 2 o 3 sio 2 k 2 o h 2 o at high fo 2 has been constructed and the effects of zno and fe 2 o 3 on this grid have been explored combining available experimental and natural occurrence data the high fo 2 invariant points in the partial grid have been located in p t space geothermobarometric data and consideration of the deduced mineral reactions in the petrogenetic grid show that the spinel granulites evolved through an anticlockwise p t trajectory reaching peak metamorphic conditions 9 kbar and 950 c followed by near isobaric cooling dt dp 150 c kbar this was superimposed by an event of near isothermal decompression dt dp 15 c kbar the studied spinel granulites therefore preserve relic prograde mineral associations and reaction textures despite being metamorphosed at very high temperatures and bear evidence of polymetamorphism

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