reappraisal of the palaeomagnetism of the miocene intramontane pag and drnis sinj basins external dinarides croatia abstract this paper presents palaeomagnetic results from the miocene offshore pag and the twin onshore drnis sinj basins earlier magnetostratigraphic results were published from both basins which documented that the lake sediments were good targets for palaeomagnetism from the pag basin we sampled the oldest and youngest segments of the 1200 m long crnika section and obtained statistically different palaeomagnetic directions from the two parts during a repeated visit to the section it was revealed that modern gravity driven creeping can account for this i e the results from the pag basin should be rejected from regional tectonic interpretation the overall mean palaeomagnetic direction for the drnis sinj basin has excellent statistical parameters its high quality is further supported by positive regional fold tilt and reversal tests based on seven geographically distributed localities the results suggests 13 20 ccw rotation with respect to africa and 21 27 with respect to stable europe during the last 15 million years as the external dinarides are the loci of a complicated network of miocene and even younger tectonic zones we cannot export the observed rotation for the whole unit but consider our results as one step in obtaining robust kinematic constraints for the post oligocene tectonic history of the external dinarides

origin of paired high pressure ultrahigh temperature orogens a ridge subduction and slab window model introductionmetamorphism at elevated pressure temperature conditions and distinct uid regimes generates ultrahigh pres sure uhp high pressure hp andultrahigh temperature uht rocks harley 2004 brown 2006 2007 2009 kelsey 2008 santosh andomori 2008a b santos et al 2009 santosh et al 2009a zhang et al 2009 although previous studies havecorrelated these di erent orogens todistinct tectonic settings recent inves tigations record high pressure anduht conditions from the same tec tonic environment within the sameorogenic belt and in some cases evenwithin the same samples mostoftheseexamples come from major collisionalzones along which continental blocksweresuturedatdi erenttimesinearthhistory including from the neoarch ean hp uht rocks of the limpopobeltinsouthafrica tsunogaeandvanreenen 2006 the palaeoproterozoichp guo et al 2004 obrien et al 2005 and uht santosh et al 2007a b 2008 2009b belts in thenorth china craton and the hp uht belts in the neoproterozoiceastern ghats belt as well as thecambrian gondwana suture in south ern india shimpo et al 2006 tsuno gaeet al 2008a santoshet al 2009c notably the thermal regimes for thesepaired hp and uht rocks are similar 1000 c and they also share acommon uid history characterizedby co

paleomagnetism of the istria peninsula yugoslavia 280 core samples were collected from upper jurassic cretaceous and eocene sediments outcropping in the istria peninsula yugoslavia due to the very low intensities of the initial natural remanent magnetizations more than 50 of the collection consisting mainly of rock samples of jurassic and eocene sediments was not suitable for paleomagnetic studies the cretaceous samples yield a mean paleomagnetic pole lat 53 long 275 and 95 4 8 which is significantly different from the african and european paleomagnetic poles of the same age the position of the istria peninsula on the autochthonous adriatic platform allows the result to be interpreted as applicable to all the autochthonous periadriatic region this new paleomagnetic result indicates that the autochthonous adriatic platform rotated counterclockwise over an angle of about 30 with respect to africa in post mesozoic times

ancient platform carbonates with well developed lofer cyclicity new candidates for magnetostratigraphy and geodynamically oriented palaeomagnetism summary young platform carbonates seem to preserve continuous records of the magnetic field in contrast ancient ones often fail to yield a meaningful palaeomagnetic signal the frequent failure of the palaeomagnetic method in the platform carbonates of the alpinemediterranean zone resulted in an investigation as to what extent the original lithology is responsible for this situation the material selected for the study represents late triassic platform facies with welldeveloped lofer cycles metre scale peritidal subtidal cycles of the transdanubian central range in hungary with evidence for minimal late diagenetic tectonic or secondary alteration processes by selecting an area where the expected declinations for the late triassic are rotated in a counterclockwise manner by a significant amount relative to the present north we hope to recognize ancient natural remanence on the basis of both consistency and departure from the earth s present field direction for the study different members of lofer cycles were drilled at two sections 13 km apart 71 and 38 samples respectively were taken from the two sections and fully oriented in the field using standard palaeomagnetic processing techniques ancient natural remanence was isolated in all a b and c members of the lofer cycle although different members were characterized by different magnetic parameters c members were diamagnetic with very low nrm intensity a members had positive susceptibility and relatively strong nrm and b members intermediate properties the mean palaeomagnetic directions isolated for the two sections based on 58 and 30 samples respectively are d 306 i 47 k 43 ay5 3 and d 300 i 52 k 26 ay5 5 before and d 31 3 i 52 and d 307 i 52 after tectonic correction both normal and reversed polarities were observed and there were several polarity reversals even in short sections these results suggest that platform carbonates with lofer cycles are likely to preserve primary remanence thus they are suitable for magnetostratigraphy studies if free from karstic cavities and fissures the presence of a members is very favourable for the preservation of the original magnetization since general water circulation is inhibited by these members but these may represent condensed intervals thus the sedimentary record in these members is not necessarily complete despite the above limitations and also the possibility of gaps at sequence boundaries platform carbonates are good candidates for future definition of magnetostratigraphic scales for example in the late triassic they also promise palaeomagnetic results of considerable tectonic importance since platform carbonates with lofer cycles are widespread in the alpine mediterranean zone

palaeomagnetism of cretaceous carbonates from the northwestern part of the dinaric fold belt abstract from the island of cres and the fold belt north of autochthonous istria 260 light grey cretaceous limestone samples were collected at 25 localities stepwise thermal demagnetization was employed to clean the remanence up to max 525 c about two thirds of the samples were successfully cleaned and the locality means were calculated from the characteristic remanences the locality means significantly deviate from the present field direction both before and after tectonic correction the mean inclination is about 15 lower than the present inclination and the declination is rotated counterclockwise i e it basically fits the pattern so far observed in the periadriatic region unfolding barely influences the scatter of the directions cres d 308 i 45 k 26 95 13 3 before tilt correction and d 330 i 48 k 33 95 11 9 after tilt correction fold belt north of autochthonous istria d 309 i 48 k 17 95 11 9 before tilt correction and d 336 i 41 k 13 95 14 0 after tilt correction in view of the inconclusive fold test the magnetization cannot be dated precisely and thus does not yield sufficient information concerning possible relative movements between autochthonous istria gargano and the fold belt studied nevertheless the counterclockwise rotated declinations of the area studied are in contrast with the clockwise rotated declinations of the ionian zone of western greece the large angular difference in declination between two parts of the dalmatian ionian zone with similar tectonic trends casts doubt on the continuity of the fold belt

paleomagnetism of external southern and central dinarides and northern albanides implications for the cenozoic activity of the scutari pec transverse zone paleomagnetic investigations have been conducted on 43 cretaceous to miocene sites 500 oriented cores from the southern and central external dinarides and from the northern external albanides the 24 reliable sites out of 33 from the dinarides do not document any significant rotation with respect to africa since at least early eocene this result contrasts with the very coherent post eocene 45 clockwise rotation previously observed in the external albano hellenides from peloponesus to the tirana basin these new results suggest that the scutari pec line located between the two orogens and considered as a mesozoic transform fault on the basis of geological observations is the place where the torsion between the dinarides and the albanides recently occurred on a local scale four sites out of 10 from two ridges located in northern albania south of the scutari pec line but with a dinaric trend show that these two structures have undergone a 30 post eocene clockwise rotation this intermediate value of rotation between the north and the south suggests that the scutari pec line is in fact a rather wide transversal zone in which the two ridges are located it acted as a decoupling zone between the dinarides and the albano hellenides during the cenozoic and probably until the pleistocene time the strong seismic activity recorded in this region and the anomalously high positive heat flow documented by other authors are also evidence for the recent activity of the scutari pec transverse zone

structural applications and discussion of a paleomagnetic post paleozoic data base for the central mediterranean abstract a post paleozoic paleomagnetic data base has been compiled for the central mediterranean with the intention of listing directions at the highest possible level of data combination the value of the entries depends on the number and quality of data available the most reliable data are those based on regionally distributed localities and sites where the age is known to a stage and where tectonic correction has been made these represent a whole tectonic unit in a given period of time the lowest quality directions are those based on few samples and isolated in space and time in the tectonic interpretation of the listed data this variation in quality of data has been taken into consideration indications for cenozoic movements having a fundamental influence on the now observable distribution of the clockwise and counter clockwise rotated paleomagnetic directions were treated first and those for older movements afterwards microplate block and nappe movements were considered as possible mechanisms for the rotation of these the last two mechanisms explained satisfactorily the occasional local deviations from a general pattern other differences however such as those which exist between the northern calcareous alps or the tisza unit and the counter clockwise rotated superunit of the central mediterranean which are large and persisted through a longer period of time are best explained by independent movement of the whole unit

controls on the locus of intraplate deformation in central australia abstract the locus of intraplate deformation in central australia changed from the presently preserved southern margin of the amadeus basin during the late neoproterozoic to early phanerozoic petermann orogeny to the northern margin in the late phanerozoic alice springs orogeny immediately prior to each event the thickness of the amadeus basin sediments varied from as little as 1 5 km in the central parts of the basin to as much as 7 km in the vicinity of the presently preserved margins of the basin with the locus of deformation mimicking the locus of maximum sedimentary thicknesses at the onset of each orogenic event as such these orogenies represent extreme examples of basin inversion a process observed in many intracratonic basins worldwide we show that differential burial of the central australian proterozoic basement complexes beneath the amadeus basin is capable of producing variations in moho temperature of up to 30 c prior to the petermann orogeny and up to 110 c prior to the alice springs orogeny for a brace goetze model of lithospheric rheology the variations in moho temperature equate to variations in effective lithospheric strain rate of 1 4 orders of magnitude implying that variations in thickness of the sedimentary blanket may have played a primary role in localising phanerozoic intraplate deformation in central australia an appealing aspect of this model lies in its corollary that the removal of the sedimentary blanket during denudation of the orogen will be accompanied by dramatic cooling and strengthening of the lithosphere this may provide a plausible explanation for the long term persistence of the extraordinary gravity anomalies 150 mgal developed during these central australian intraplate orogenies

new constraints on metamorphism in the rauer group prydz bay east antarctica granulite facies metapelites of the mather and filla paragneisses within the rauer group east antarctica possess markedly different compositions the metamorphic evolution of the two metapelite types has been interpreted as temporally distinct with the rauer group preserving at least two distinct granulite facies tectonothermal episodes calculated p t pseudosections and orthopyroxene al content indicate the revised maximum preserved p t conditions within the mather paragneiss to lie in the vicinity of 950 975 c and 10 10 6 kbar less extreme than previous estimates the range of possible p t paths for the mather paragneiss consistent with mineral textural relationships and pseudosections contoured for mineral proportion are significantly shallower dp dt than previous estimates a near isothermal decompression p t path and extreme peak metamorphic conditions are not necessary to explain the development of preserved mineral reaction textures the filla paragneiss contains pelitic assemblages less amenable to rigorous quantitative analysis nevertheless possibilities for the shared or otherwise metamorphic evolution of the mather and filla paragneisses may be postulated on the basis of calculated pseudosections in the context of existing geochronology for the rauer group and preserved microstructures a shared evolution most likely during pan african granulite facies metamorphism is plausible and consistent with mineral assemblage development geochronology and microstructures a revised interpretation of the rauer group s preserved metamorphic evolution may warrant the revision of existing tectonic models applicable also to the remainder of prydz bay more generally the employed approach may incite a revision of peak p t and p t paths in other granulite facies terranes

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

first application of the revised ti in zircon geothermometer to paleoproterozoic ultrahigh temperature granulites of tuguiwula inner mongolia north china craton the revised titanium in zircon geothermometer was applied to paleoproterozoic ultrahigh temperature uht granulites at tuguiwula inner mongolia north china craton the tuguiwula granulites contain diagnostic uht mineral assemblages such as sapphirine quartz and high alumina orthopyroxene sillimanite quartz suggesting formation under temperatures of ca 1 000 c and pressures of up to 10 kbar here we report detailed petrographic studies and icp ms data on titanium concentration in zircons associated with the uht assemblages the zircons associated with sapphirine spinel sillimanite magnetite assemblages have the highest ti concentration of up to 57 ppm yielding a temperature of 941 c and suggesting that the growth of zircons occurred under ultrahigh temperature conditions the maximum temperatures obtained by the revised ti in zircon geothermometer is lower than the equilibrium temperature of sapphirine quartz indicating an interval of cooling history of the granulites from uht condition to ca 940 c many of the zircons have ti concentrations ranging from 10 to 33 ppm indicating their growth or recrystallization under lower temperatures of ca 745 870 c these zircons are interpreted to have recrystallized during the retrograde stage indicated by microstructures such as cordierite rim or corona between spinel and quartz and orthopyroxene cordierite symplectite around garnet previous geochronological study on the zircons of the tuguiwula uht granulites gave a mean u pb shrimp age of 1 92 ga however based on the ti in zircon geothermometer results reported in this work and considering the relatively slow thermal relaxation of these rocks we infer that the timing of peak uht metamorphism in the tuguiwula area could be slightly older than 1 92 ga

shrimp u pb zircon geochronology of palaeoproterozoic metasedimentary rocks in the north china craton evidence for a major late palaeoproterozoic tectonothermal event abstract although metasedimentary successions are widely distributed across the north china craton those of palaeoproterozoic age are restricted to three orogenic belts namely the trans north china orogen the khondalite belt and the jiao liao ji belt prior to this work some of those successions were considered to be archaean in age the palaeoproterozoic metasedimentary rocks contain detrital igneous zircon grains with ages ranging from 3 0 to 2 1 ga with the majority recording an age between 2 3 and 2 0 ga this latter group define a series of magmatic events that occurred between formation of the archaean basement and deposition of the metasedimentary rock protoliths the metasedimentary rocks occur as two associations older volcano sedimentary units that formed between 2 37 and 2 0 ma and younger sedimentary units that formed mainly between 2 0 and 1 88 ga all these rock associations were then subjected to metamorphism during the late palaeoproterozoic 1 88 and 1 85 ga tectonothermal event named the luliang movement in early chinese literature which was a continent continent collision that led to the amalgamation of the north china craton

episodic paleoproterozoic 2 45 1 95 and 1 85 ga mafic magmatism and associated high temperature metamorphism in the daqingshan area north china craton shrimp zircon u pb dating and whole rock geochemistry abstract in the daqingshan area within the khondalite belt of the north china craton paleoproterozoic gabbro and dolerite intrusions show amphibolite to granulite facies metamorphism u pb zircon dating and whole rock geochemistry were undertaken on these rocks in order to understand the timing of mantle magmatism and thermal processes ten samples yielded zircon u pb formation ages of 2 46 2 44 ga 1 97 1 92 ga and 1 84 ga and metamorphic ages of 1 95 1 83 ga they are variable in geochemical composition with some being enriched in light rare earth and large ion lithosphere elements but depleted in nb ta combined with previous results it is concluded that mafic magmatism at 2 45 2 37 ga and 1 97 1 92 ga indicates two paleoproterozoic extensional phases with the latter accompanied by ht uht metamorphism as a result of underplating of mantle magma

the tectonic development of the northern adriatic region constrained by jurassic and cretaceous paleomagnetic results abstract in the northern adriatic region there is a clear boundary between the weakly deformed stable core of the adriatic microplate and its tectonically complicated ne margin the external dinarides which are further subdivided on stratigraphic and structural grounds although most authors distinguish an adriatic and a dinaridic realm within the external dinarides the relationship and the present boundary between the realms are matters of discussion the aim of this study was to obtain paleomagnetic directions from different parts of the northern adriatic segment of the external dinarides and discuss their bearing on the different types of the tectonic models for this purpose we carried out standard paleomagnetic measurements on biostratigraphically well controlled samples from 28 localities from the adriatic mainland and the northern adriatic islands and from one middle jurassic locality representing the stable core of the adriatic microplate in addition six localities from the northern adriatic islands earlier studied paleomagnetically were biostratigraphically updated and thus included in the data set for the northern adriatic islands an albian d i 333 48 k 44 95 9 and a cenomanian santonian d i 334 46 k 188 95 5 paleomagnetic direction was defined they are in perfect agreement with coeval paleomagnetic directions from stable adria thus the paleomagnetic data clearly support the models which conceive the area as the imbricated margin of the adriatic microplate the early d i 338 49 k 118 95 11 the middle d i 342 54 k 112 95 7 and the late d i 336 42 k 62 95 16 jurassic paleomagnetic directions all representing the tectonic units of the adriatic mainland suggest an about 30 cw rotation of this belt of the external dinarides with respect to stable adria the difference can be interpreted as inherited from the differencial rotations of two independent carbonate platforms an adriatic and a dinaric alternatively thrusting of the more internal belt of the external dinarides above the adriaticum may be responsible for the difference

paleomagnetism of the kvarner islands yugoslavia abstract nearly 400 samples were drilled at 44 localities from cretaceous and eocene carbonates and eocene flysch all in para autochthonous position as a result of thermal cleaning most of the cretaceous localities yielded statistically well defined mean directions thus overall means were calculated for the cretaceous of cres and krk islands respectively cres six localities d 308 i 45 k 26 95 13 3 d c 330 i c 48 k 33 95 11 9 krk seven localities d 342 i 42 k 12 95 17 8 d c 348 i c 48 k 26 95 12 1 these directions corrected for tilt agree well with those determined for rocks of similar age from the fold belt north of autochthonous istria the trieste karst and the southern alps while exhibiting clockwise rotation of about 30 with respect to the rigid part of the adriatic region the natural remanent magnetization nrm of most of the eocene sediments was either too weak to be cleaned properly or became erratic before characteristic remanent magnetization chrm could be isolated the mean directions of the eocene localities with chrm differ so much both before and after tectonic correction that an overall mean was not calculated despite the scatter certain trends could be recognized counter clockwise rotation characterizes most of the eocene sites the two sites in krk with clockwise rotation may belong not to the dalmatian but the high karst zone

counterclockwise rotation of the western alps since the oligocene new insights from paleomagnetic data 1 a paleomagnetic study of penninic units in the southern part of the alpine arc has been carried out more than 200 samples 23 sites were collected in brianconnais ammonitico rosso limestones of the high ubaye valley and the ligurian alps a characteristic component of magnetization of reverse polarity was isolated on most of the sites this component does not pass the fold test and is interpreted as a tertiary overprint related to alpine metamorphism mean directions in geographic coordinates are d 121 i 52 95 11 and d 72 i 48 95 15 for the ubaye and the liguria localities respectively indicating large counterclockwise rotations about vertical axis of 68 and 117 relative to stable europe these rotations in agreement with a previous study conducted in the briancon area together with other paleomagnetic data from the western alps show that the internal alps suffered a large but nonhomogeneous counterclockwise rotation since the oligocene the rotations are in agreement with the combination of earlier separately proposed processes the rotation of the adriatic plate accounts for about 25 of rotation remaining rotation and southward gradient would be related to left lateral shear accommodating the displacement between adria and europe at the southern border of the western alps furthermore the southward extrusion of the western alps south of the simplon fault zone may account for up to 10 of rotation rotations appear therefore as a major process accommodating deformation in the western alps since the oligocene

is the ordos block archean or paleoproterozoic in age implications for the precambrian evolution of the north china craton the ordos block is a large but poorly exposed crustal segment in the western part of the north china craton little is known about its precambrian basement rocks and their evolution because of an extensive cover by younger sedimentary rocks we report shrimp zircon ages and hf in zircon isotopic compositions for five samples recovered from drill holes that penetrated into the ordos basement based on these data two age groups of rocks can be distinguished an early paleoproterozoic group that is represented by one sample from the northern margin of the ordos block and a late or middle late paleoproterozoic group consisting of the remaining four samples the early paleoproterozoic sample contains zircon cores and metamorphic rims with ages of 2 4 ga and 2 28 ga respectively the cores have ehf t values of 8 8 to 6 2 and hf crustal model ages of 2606 to 3221 ma the remaining rocks of late or middle late paleoproterozoic age contain zircons that may also show a core rim structure but with the cores having ages of 2 08 ga and the metamorphic rims recording ages of 1 9 ga the zircon cores have ehf t values of 9 1 to 10 1 and hf crustal model ages of 2039 to 3068 ma combined with data from earlier studies we draw the conclusion that late paleoproterozoic metasedimentary rocks are widespread in the ordos basement there is no evidence from our samples of an archean component and this calls into question the prevailing view that the ordos basement is an archean cratonic block the fact that the ordos basement was involved in a widespread late paleoproterozoic tectono thermal event indicates that most current models for the tectonic evolution of the western portion of the north china craton need to be revised

p t t paths and tectonic history of an early precambrian granulite facies terrane jining district south east inner mongolia china abstract the widespread khondalite series of south east inner mongolia consists largely of biotite sillimanite garnet gneiss and quartzo feldspathic gneiss with some marble and mafic granulite layers it has experienced two metamorphic events at c 2500 and 1900 2000 ma a pre peak stage of the first metamorphism at t 600 700 c and p 6 7 kbar is recognized by the relict amphibolite facies assemblage ky grt bt pl qtz and protected inclusions of biotite hornblende sodic plagioclase and quartz in garnet or orthopyroxene the peak stage with t c 800 50 c and p 8 10 kbar is characterized by the widespread granulite facies assemblages sil grt bt kfs pl qtz in gneiss and opx cpx pl hbl grt in granulite the p t t path suggests that the supracrustal sequence was buried in the lower crust by tectonic thickening during d1 d2 the beginning of the second metamorphism is characterized by further temperature rise to 700 c or more at lower pressure this stage is manifested by the appearance of cordierite after garnet fibrolite sil2 after biotite in gneiss and transformation of hbl1 into opx2 and cpx2 in granulite coronas of symplectitic opx2 pl2 surrounding grt1 and cpx1 in mafic granulite are interpreted as products of near isothermal decompression the p t t path may be related tectonically to waning extension of the crust by the end of the early proterozoic

sapphirine bearing granulites and related high temperature metamorphic rocks from the higo metamorphic terrane west central kyushu japan the higo metamorphic unit in west central kyushu island southwest japan is an imbricated crustal section in which a sequence of units with increasing metamorphic grade from high northern part to low southern part structural levels is exposed the basal part of the metamorphic sequence representing an original depth of 23 24 km consists mainly of garnet cordierite biotite gneiss garnet orthopyroxene gneiss orthopyroxene bearing amphibolite and orthopyroxene bearing s type tonalite these metamorphic rocks underwent high amphibolite facies up to granulite facies metamorphism with peak p t conditions of 720 mpa 870 c in addition sapphirine bearing granulites and related high temperature metamorphic rocks also occur as tectonic blocks in a metamorphosed peridotite intrusion the sapphirine bearing granulites and their related high temperature metamorphic rocks can be subdivided into five types of mineral assemblages reflecting their bulk chemical compositions as follows 1 sapphirine corundum spinel cordierite 2 corundum spinel cordierite 3 garnet corundum spinel cordierite 4 garnet spinel gedrite corundum and 5 orthopyroxene spinel gedrite these metamorphic rocks are characterized by unusually high al2o3 and low sio2 contents which could represent a restitic nature remaining after partial melting of pelitic granulite under the ultra high temperature contact metamorphism at the peak metamorphic event of the higo metamorphic unit the metamorphic conditions are estimated to be about 800 mpa and above 950 c which took place at about 250 ma as a result of the thermal effect of the regional gabbroic rock intrusions

geochronology of galapagos seamounts lavas from seamounts of the central galapagos platform and the carnegie ridge increase in age with distance from the western edge of the platform consistent with a hotspot model for the generation of these features the areal distribution of seamount ages suggests that the pattern of dispersed volcanism seen on the present islands also prevailed between 5 and 6 ma age distance relationships are consistent with a decrease in the velocity of the nazca plate relative to the galapagos hotspot during the past 9 m y most likely at 5 ma when there was a change in pacific hotspot motion alternatively the plate velocity remained constant and volcanism at a single seamount endured up to 7 m y lavas from seamounts along the wolf darwin lineament to the northwest of the galapagos platform are young 1 ma with the youngest measured ages closest to the platform

thermal evolution rate of exhumation and tectonic significance of metamorphic rocks from the floor of the alboran extensional basin western mediterranean high grade metamorphic rocks drilled at ocean drilling program site 976 in the alboran sea show a pt path characterized by decompression from about 1050 mpa 40 km depth to 350 mpa 13 km depth accompanied by an increase in temperature from about 550 50 c to 675 25 c the ar ar dating on muscovite and apatite fission track analysis indicate that the final stage of exhumation and cooling occurred very rapidly in the interval 20 5 18 ma which coincides with the initiation of sedimentation in the alboran sea basin the alboran sea formed by miocene extension on the site of a late cretaceous to paleogene contractional orogen and extension coincided with thrusting in the peripheral parts of the betic rif arc which surrounds the basin on three sides thermal modeling of the pt path was carried out with the aim of constraining geodynamic models for the formation of the basin variables considered in the modeling included 1 the thickness and thermal gradient of the postorogenic lithosphere 2 the radiogenic heat production in the thickened crust 3 the time gap pause between the end of contractional tectonics and the start of extension 4 removal of lithospheric mantle below 125 75 or 62 5 km and 5 the rate of extension the only combinations of variables that produce modeled pt paths with the observed characteristics involve high radiogenic heat production combined with a significant postcontractional pause to produce high temperatures in rocks initially at 40 km depth removal of lithosphere below 62 5 km to produce further heating during decompression extension by a factor of 3 in 6 m y to delay the attainment of the maximum temperature until the rocks reached shallow depths and final exhumation and cooling in about 3 3 m y to satisfy radiometric and petrological constraints this gives a maximum of about 9 m y for exhumation from 40 km depth to the surface lithospheric stretching in response to plate boundary forces such as trench rollback without removal of lithosphere cannot explain the late onset of heating and the high temperatures reached by these rocks removal of lithosphere at depths significantly greater than 62 5 km cannot explain the combination of high temperatures reached by these rocks and the shallow depth at which they attained the maximum temperature only a combination of significant postcollisional radiogenic heating then wholesale removal of lithospheric mantle below the orogenic crust followed by rapid stretching can explain the observed pt path these results appear to support some form of lithospheric delamination as the primary cause for the formation of the alboran sea basin

carbonic metamorphism at ultrahigh temperatures evidence from north china craton abstract the north china craton ncc constitutes one of the fundamental precambrian nuclei of asia the paleoproterozoic khondalite belt along the northern margin of the ncc shows evidence for extreme crustal metamorphism at temperatures exceeding 1000 c and pressures of 10 12 kbar with the formation of diagnostic ultrahigh temperature uht mineral assemblages including sapphirine quartz high alumina orthopyroxene sillimanite and high temperature mesoperthites here we record for the first time the common occurrence of co2 rich fluid inclusions in peak uht minerals in the mg al granulites of ncc and trace the fluid evolution history from microthermometric and laser raman spectroscopic studies we show that the syn metamorphic fluid in the uht rocks was pure co2 while very high density carbonic fluids are preserved in some cases corresponding to entrapment at peak conditions many of the inclusions underwent density modification during the rapid and near isothermal exhumation of the rocks following peak uht metamorphism and initial cooling at depth the present study area within the northern margin of the ncc defines a collisional orogenic belt where continental collision and deep subduction were possibly followed by extensional collapse of the orogen evidence for the extensional phase is provided by the several mafic magmatic intrusions of paleoproterozoic age that are seen emplaced within the granulite facies supracrustals the thermal anomaly displayed by the uht granulites may thus correlate with asthenospheric upwelling and emplacement of mafic magmas possibly associated with plume activity which provided the heat and co2 input to achieve extreme crustal metamorphism and generation of dry granulite facies assemblages our study is the first report of syn metamorphic co2 rich fluids in uht granulites from a paleoproterozoic ca 1 92 ga terrane and provide evidence for carbonic metamorphism at ultrahigh temperatures as north china craton joined the columbia supercontinent amalgam

hot orogens tectonic switching and creation of continental crust many granulite terrains were too hot to have formed during continental collision rather along with many high grade metamorphic terrains that typify continental crust most formed in accretionary orogens during tectonic switching when prolonged lithospheric extension was interrupted by intermittent transient contraction based on modern and ancient examples tectonic switching occurs when slab retreat induces upper plate extension causing arc splitting formation of microcontinent slivers and backarc basins then intermittent arrival of buoyant oceanic plateaus induces transient flat subduction or slab flip and crustal thickening during slab retreat basaltic magmas produced from decompressed asthenosphere advect into the extending orogen causing granulite facies metamorphism and granite generation but subsequent thickening during flat subduction cools the region thickening is focused in the thermally softened backarc region and if sediment filled a hot short lived 10 m y narrow 50 100 km orogenic belt forms such thickening is often misleadingly ascribed to arc or microcontinent collision once slab retreat mode is reestablished lithospheric extension recommences and a new arc backarc system forms generally outboard arrival of another plateau will reverse the procedure and another short lived hot orogen will form within the orogenic system cycles of tectonic switching efficiently produce continental crust

paleomagnetic evidence for cenozoic clockwise rotation of the external albanides abstract a paleomagnetic study of 750 samples obtained from 55 late eocene to middle pliocene sedimentary sites demonstrates a clockwise rotation of about 45 of the external zones of albania this rotation occurred either in two phases of roughly equal amplitude during the early middle miocene and plio pleistocene or in a single phase with a clear acceleration of the movement during the plio pleistocene the good agreement of these results with those from southern albania and northwestern greece indicates that a large geographical area is involved in the rotation the active aegean compressive front thus extends much farther north than hitherto believed and the rotational pattern is not significantly distorted by the tectonic discontinuities recorded by geological studies the external zones of the albano hellenic belt appear to have rotated virtually as a single entity from the peloponnesos to northern albania over a deep decollement level probably involving the basement itself