passive and active margin history of the northern tatricum western carpathians slovakia the tatricum an upper crustal thrust sheet of the central western carpathians comprises pre alpine crystalline basement and a late paleozoic mesozoic sedimentary cover the sedimentary record indicates gradual subsidence during the triassic early jurassic initial rifting a jurassic early cretaceous extensional tectonic regime with episodic rifting events and thermal subsidence periods and middle cretaceous overall flexural subsidence in front of the orogenic wedge prograding from the hinterland passive rifting led to the separation of the central carpathian realm from the north european platform a passive margin rimmed by peripheral half graben was formed along the northern tatric edge facing the vahic south penninic oceanic domain the passive versus active margin inversion occurred during the senonian when the vahic ocean began to be consumed southwards below the tatricum it is argued that passive to active margin conversion is an integral part of the general shortening polarity of the western carpathians during the mesozoic that lacks features of an independent wilson cycle an attempt is presented to explain all the crustal deformation by one principal driving force the south eastward slab pull generated by the subduction of the meliatic triassic jurassic tethys oceanic lithosphere followed by the subcrustal subduction of the continental mantle lithosphere

mid cretaceous primitive alkaline magmatism in the northern calcareous alps significance for austroalpine geodynamics basanitic dykes local name ehrwaldite occur over a distance of at least fifty kilometres in a narrow east west trending zone within the lechtal nappe of the northern calcareous alps the dykes crosscut mesozoic sedimentary strata up to the lower cretaceous the sedimentary rocks show contact metamorphism in the immediate vicinity of the dykes the ehrwaldites vary in mineral content from olivine fo 91 87 titanian augite basanites to basanites containing also kaersutite titanian biotite and zeolites minor phases are titanian magnetite picotite llmenite carbonate and sulfides mantle derived xenoliths and xenocrysts form up to five percent of the rock bulk chemical analyses show the typical characteristics of nepheline basanites ne 11 17 wt mg number 74 78 and demonstrate that the ehrwaldites are primitive mantle derived melts trace element contents of the dykes are in excellent agreement with those for basanites and olivine nephelinites from tertiary and quaternary european provinces the same is true for the isotope characteristics of the dykes with average initial esr and end values of 8 2 and 4 7 respectively these data demonstrate similarities in the mantle source rocks and in the melting characteristics with european alkaline rocks and thus similarities in the european subcontinental mantle at different localities for the last 100 ma these data are in clear contrast with the value for the penninic suboceanic mantle geochemical constraints and piston cylinder experiments indicate a depth of origin of 80 kilometres at temperatures of at least 1250 c for the ehrwaldites potassium argon bulk rock data of petrographically different samples of the dykes from different localities yield consistent ages around 100 ma upper albian after emplacement the ehrwaldites underwent conditions of the zone of diagenesis with temperatures around 120 c which the late magmatic zeolites survived

derivation of mesozoic adakitic magmas from ancient lower crust in the north china craton three mesozoic plutons were intruded into the archean granulite terrains in northern margin of the north china craton adjacent to the granulite xenolith bearing tertiary hannuoba basalts they are triassic guzuizi and honghualiang granites and early cretaceous zhuanzhilian diorite rocks of the three intrusions are characterized by high sr 576 1216 ppm and na2o 4 0 low y 18 ppm and heavy rare earth elements yb 1 8 ppm these features are similar to modern adakites siliceous rocks with high sodium aluminum and strontium and low heavy rare earth element and yttrium contents from island arcs and archean high al tonalite trondhjemite granodiorite ttg however they are distinctly potassium rich and their evolved strontium neodymium and hafnium isotopic compositions and inherited zircon ages coincide with those of the archean granulites and the hannuoba granulite xenoliths such features cannot readily be explained with previous models of the origin of adakites and ttgs we propose that these adakitic rocks are formed by partial melting of ancient lower crust and that the restites are represented by some of the hannuoba granulite xenoliths we further argue that this could be one major mechanism to generate voluminous mesozoic adakitic magmas in the eastern north china craton our hypothesis can also explain the discrepancies in ages and compositions between the granulite terrains and xenoliths and implies that crustal anatexis may be one of the major processes controlling the chemical differentiation of the continental crust

distinct variations of crustal shear wave velocity structure and radial anisotropy beneath the north china craton and tectonic implications article i nfo this study presents the crustal shear wave velocity structure and radial anisotropy along two linear seismic arrays across the north china craton ncc from ambient noise tomography about a half to one year long ambient noise data from 87 stations were used for obtaining the inter station surface wave empirical green s functions egfs from cross correlation rayleigh and love dispersion curves within the period band 5 30 s were measured from the egfs of the vertical and transverse components respectively these dispersion data were then used to determine the crustal shear wave velocity structure vsv and vsh and radial anisotropy 2 vsh vsv vsh vsv from point wise linear inversion with constraints from receiver function analysis our results reveal substantial structural variations among different parts of the ncc the bohai bay basin in the eastern ncc is underlain by a thin crust 30 km with relatively low velocities particularly vsv and large positive radial anisotropy in the middle to lower crust such a crustal structure is no longer of a cratonic type and may have resulted from the widespread tectonic extension and intensive magmatism in this region since late mesozoic beneath the ordos basin in the western ncc the crust is relatively thicker 40 km and well stratified and presents a large scale low velocity zone in the middle to lower crust and overall weak radial anisotropy except for a localized lower crust anomaly the overall structural features of this region resemble those of typical precambrian shields in agreement with the long term stability of the region the crustal structure under the trans north china orogen tnco central ncc is more complicated and characterized by smaller scale velocity variations strong positive radial anisotropy in the middle crust and rapid change to weak to negative anisotropy in the lower crust these features may reflect complex deformations and crust mantle interactions probably associated with tectonic extension and magmaticunderplating during the mesozoic to cenozoic evolution of the region our structural images in combination with previous seismic geological and geochemical observations suggest that the phanerozoic lithospheric reactivation and destruction processes may have affected the crust especially the middle and lower crust of the eastern ncc and the effect probably extend ed to the tnco but may have minor influence on the crust of the western part of the craton

phanerozoic magma underplating associated with remelting of the lower crust beneath the cathaysia block evidence from zircon upb ages and hfo isotopes of granulite xenoliths from daoxian south china abstract widespread mesozoic granitoids in the cathaysia block of south china are associated with intensive reworking of the lower crust as a result of magma underplating this inference is based mainly on studies of mafic igneous rocks whereas there is little evidence from lower crustal rocks lower crustal xenoliths in mesozoic basalts in the daoxian region within the cathaysia block might record information on the relationship between magma underplating and remelting of the pre existing crust beneath the block the xenoliths are mainly mafic granulites with minor felsic granulites the mafic granulites have low sio2 contents 47 22 49 46 wt and high mg values 77 8 82 4 suggesting that their protoliths were derived from a mantle source the felsic granulite xenoliths have high sio2 69 56 70 27 wt and low mgo 1 63 2 50 wt contents and zircons in these xenoliths yield negative ehf t values 6 1 to 12 6 and high 18o values 6 8 7 6 consistent with a crustal source both mafic and felsic granulite xenoliths record magmatic 226 218 ma and metamorphic 218 193 ma events suggesting a genetic link between mafic and felsic rocks in the lower crust we propose that the magma underplating was responsible for the origin of the mafic granulites and partial melting of pre existing lower crust as recorded by the felsic xenoliths the granulite xenoliths also contain abundant inherited zircons that formed during the archean neoproterozoic 2584 659 ma early paleozoic peaking at ca 425 ma and late paleozoic peaking at ca 261 ma zircons from the daoxian granulite xenoliths have distinct hf o isotopic compositions that record the multistage evolution of the lower crust beneath the cathaysia block this evolution involved the accretion of juvenile crust during the late archean ehf t 4 2 to 4 6 and late paleozoic ehf t 1 3 to 5 3 18o 5 8 6 2 crustal reworking during the neoproterozoic ehf t 7 5 to 11 8 18o 5 1 9 5 and early paleozoic ehf t 0 5 to 2 2 18o 7 3 7 5 and crustal accretion with significant reworking during the early mesozoic ehf t 19 2 to 5 9 18o 6 8 7 6 the u pb ages and hf o isotopic data of zircons from the daoxian granulite xenoliths are consistent with the distribution of phanerozoic igneous rocks in the cathaysia block these data support the inference that mesozoic magma underplating triggered the remelting of pre existing crustal materials and produced extensive granitoid magmatism

origin of a mesozoic granite with a type characteristics from the north china craton highly fractionated from i type magmas we report geochronological geochemical and isotopic data for the mesozoic shangshuiquan granite from the northern margin of the north china craton the granite is highly fractionated with sio2 74 occurrence of annitic biotite high contents of alkalis k2o na2o rb y nb and heavy rare earth elements high feot mgo low contents of cao al2o3 ba and sr and large negative eu anomalies makes it indistinguishable from typical a type granites a mantle derived origin for the rocks of the granite is not favored because their high initial 87sr 86sr 0 706 and low end t 15 are completely different from either those of the lithospheric or asthenospheric mantle in fact their sr nd isotopes fall within the range of sr nd isotopic compositions of the archean granulite terrains and are comparable to those of mesozoic crustal derived i type granitoids in the region therefore the shangshuiquan granite is considered to be dominantly derived from partial melting of the ancient lower crust its parental magmas prove to be similar to i type magmas and to have undergone extensive fractionation during its ascent this is supported by the fact that some of the nearby hannuoba feldspar rich granulite xenoliths can be indeed regarded as the early cumulates in terms of their mineralogy chemistry sr nd isotopes and zircon u pb ages and hf isotopes it is furthermore argued that some of highly fractionated granites worldwide especially those with a type characteristics and lacking close relationship with unfractionated rocks may in fact be fractionated i type granites this suggestion can explain their close temporal and spatial associations as well as similar sr nd isotopes with i type granites our study also sheds new light on the petrogenesis of deep crustal xenoliths

the composition and petrogenesis of the lower crust a xenolith study granulite facies lower crustal xenoliths from a single basaltic vent hill 32 in the mcbride volcanic province north queensland australia illustrate the extreme lithologic diversity of the deep crust these xenoliths are dominantly mafic but intermediate and felsic granulites represent a significant proportion 20 of the xenolith population all xenoliths have high grade mineralogies and generally well equilibrated textures as well as decompression features which are indicative of derivation from deep crustal levels 0 7 1 0 gpa major and trace element chemistry for 12 xenoliths chosen to span the observed lithologic range are used to constrain the petrogenesis of these rocks in the smaller layered samples metamorphic differentiation may lead to nonrepresentative sampling however in most instances such secondary processes can be identified and the original chemical characteristics delineated the mafic xenoliths formed through a variety of processes including crystallization of mafic magma crystal accumulation from mafic and felsic magmas and partial melting of intermediate compositions to yield a mafic residuum the two analyzed intermediate xenoliths are metasediments based upon high alumina to alkali ratios and rare earth element patterns whereas the two felsic xenoliths have compositions similar to igneous rocks comparisons of noncumulate and or restite xenoliths with unmetamorphosed rock types show that k and rb are variably depleted whereas th and u are strongly depleted in all rock types the large ion lithophile element depletion patterns for xenoliths are similar to those of rocks from granulite facies terrains suggesting similar processes were operative suggestions of an anomalously ba and sr rich lower crust are not supported by the data by using the observed lithologic proportions of xenoliths at hill 32 and the chemical analyses presented here a weighted mean composition of the lower crust can be obtained and compared with recently proposed lower crustal compositions the weighted mean composition is mafic and formed through a combination of basaltic underplating crystal accumulation tectonic underplating and partial melting no simple model of lower crust formation through basaltic or andesitic underplating nor intracrustal melting is sufficient to explain lower crust formation in phanerozoic continental margin settings

phanerozoic magma underplating and crustal growth beneath the north china craton evidence for post archaean crustal growth via magma underplating is largely based on u pb dating of zircons from granulite facies xenoliths however whether the young zircons from such xenoliths are genetically related to magma underplating or to anatexis remains controversial the lower crustal xenoliths carried by igneous rocks in the chifeng and ningcheng north china craton have low sio2 and high mgo indicating that parental melts of their protoliths were of unambiguous mantle origin the xenoliths contain abundant magmatic zircons with late palaeozoic ages and have more radiogenic zircon hf isotope compositions and hence younger model ages than ancient crustal magmas and the reworking array of the basement rocks our data suggest that the granulites represent episodic magmatic underplating to the lower crust of this craton in phanerozoic time considering the observation that regional lowermost crust 5 km is mafic and characterized by phanerozoic zircons this work reports an example of post archaean crustal growth via magma underplating

mantle and crust interaction scenario at the crust mantle transition zone depicted from inter layered pyroxenite granulite xenolith in hannuoba area north china craton abstract mantle crust interaction occurring in various ways has been proposed for a long time but the details of its mechanism at the crust mantle transition zone and the lower crust are poorly known this study reports a layered pyroxenite granulite xenolith from the cenozoic basalts of hannouba in the north china craton petrological and mineral chemical investigations reveal that the pyroxenite layers are cumulates from underplating basaltic magmas while the granulite portions represent pre existing lower crust the magmas were crustally contaminated by interaction with the granulite orthopyroxene and plagioclase in the granulite involving reaction with exotic melts are transformed to k feldspar clinopyroxene fe and mg oxide melt and or sulfide the mineral chemical similarities and overlapping temperature estimates suggest that the pyroxenites and the granulites had been incrementally equilibrated with each other both physically and chemically via significant chemical and thermal exchange the phase transformation and interaction could represent crust anatexis to generate more felsic magmatism and related ore deposits for the middle to upper crust and subsequently increase crustal maturity this is also an efficient way of forming crust mantle transition zone below the lower crust and crustal reworking in the hannuoba area and may be applicable to other places worldwide

mesozoic crust mantle interaction beneath the north china craton a consequence of the dispersal of gondwanaland and accretion of asia we present evidence from zircons entrained within lower crustal xenoliths in the cenozoic hannuoba basalt of multiple melting events beneath the north china craton in the late mesozoic peak activity was between 180 and 80 ma the upper crustal signature of which was the generation of voluminous granitoids and related volcanic rocks emplacement of dioritic and lamprophyric dikes and widespread gold mineralization the process involved partial loss of mantle lithosphere accompanied by wholesale rising of asthenospheric mantle beneath eastern china we correlate these events with lithospheric thinning resulting from the breakup and dispersal of gondwanaland accompanied by a major mantle overturn fueled by the destruction of oceanic lithosphere and triggered by its sinking into the lower mantle during the subsequent accretion of asia

lithological and age structure of the lower crust beneath the northern edge of the north china craton xenolith evidence abstract deep seated xenoliths in volcanic rocks offer direct glimpses into the nature and evolution of the lower continental crust in this contribution new data on the u pb ages and hf isotopes of zircons in six felsic granulite xenoliths and one pyroxenite xenolith from the hannuoba cenozoic basalts combined with published data from mafic to felsic xenoliths are used to constrain the lithological and age structure of the lower crust beneath the northern edge of the north china craton two newly reported felsic granulites contain precambrian zircons with positive 7 5 10 6 and negative e hf values 10 1 to 3 7 corresponding to upper intercept ages of 2449 62 ma and 1880 54 ma respectively indicating crustal accretion in the late archean and reworking in paleoproterozoic time zircons in another four felsic xenoliths give phanerozoic ages from 142 ma to 73 ma and zircons from one pyroxenite xenolith give a concordant age of 158 ma the zircon e hf values of these four felsic xenoliths range between 23 3 and 19 1 reflecting re melting of the pre existing lower crust integration of geothermobarometric and geochronological data on the hannuoba xenoliths with seismic refraction studies shows that the lower crust beneath the northern edge of the north china craton is temporally and compositionally zoned the upper lower crust 24 33 km consists dominantly of archean 2 5 ga with minor 2 7 ga felsic granulites with subordinate felsic granulites that reworked at 140 120 ma both precambrian and late mesozoic mafic granulites are important constituents of the middle lower crust 33 38 km major late mesozoic 140 120 ma and less cenozoic 45 47 ma granulites and pyroxenites are presented in the lowermost crust 38 42 km the zoned architecture of the lower crust beneath hannuoba suggests a complex evolution beneath the northern margin of the craton including late neoarchean 2 5 ga accretion and subsequent episodic accretion and or reworking during paleoproterozoic 1 8 1 9 ga and phanerozoic 220 ma 120 140 ma 45 47 ma times with possible links to circum craton subduction collision events and the destruction of the north china craton

supercooled melt inclusions in lower crustal granulites as a consequence of rapid exhumation by channel flow abstract here we report on the unexpected occurrence of felsic granitic inclusions with quench textures such as spherulite and dendrite hereafter referred to as felsite inclusions similar to some volcanic rocks within garnet in presumably slowly cooled lower crustal granulites of various geologic ages ranging from early proterozoic to middle paleozoic and wide global distribution the limpopo belt the grenville province the lutzow holm complex of east antarctica the highland complex of sri lanka and the southern bohemian massif the well preserved textures of felsite inclusions are indicative of melts formed by anatexis during high pressure and high temperature metamorphism crystallization under far from equilibrium conditions at 50 c undercooling and subsequent rapid cooling the occurrence of felsite inclusions in granulites in restricted tectonostratigraphic zones in sri lanka among other examples may be the first geologic evidence for fast exhumation of lower crustal rocks to andalusite stable upper crustal conditions by channel flow in a continental collision orogen we hypothesize that granulites ascend episodically along discrete high strain zones and cool as fast as some felsic magmas this conclusion sheds new light on the debate regarding the deep crustal processes and necessitates changes to fundamental beliefs about exhumation rates based on rates of plate convergence 1 10 cm year

transition from convergence to escape field evidence from the west carpathians abstract the tertiary deformation of the west carpathians occurred during northeast directed convergence of the european and apulian plates and was complicated by space constraints imposed by orogen advance palaeostress fields computed from fault slip data and structural interpretation of known gravimetric magnetic seismic palaeomagnetic lithostratigraphic sedimentological fission track and borehole data indicate that during deformation the orogen consisted of both frontal thrust stacking zones and internal extensional trans tensional zones synthesis of available data indicates that the episodes of tectonism in litho tectonic units were both time and plate position related and were caused by conditions which varied in different areas ranging from the foreland to back arc region in each time period discrete deformation phases record the transition from convergent to tectonic escape tectonisrn principal horizontal stress trajectories were approximately co linear with the plate convergence vectors prior to escape the onset of escape conditions is indicated by 1 clockwise rotation of palaeostress orientations during the time of known escape in the eastern alps and 2 the cessation of thrusting of the flysch belt nappes onto the neogene foredeep in the westernmost part of the west carpathians tectonic escape along strike slip faults subparallel to the collision suture zone began in early badenian time in the west carpathians various regions along the orogenic belt were affected by escape that became progressively younger towards the east this agrees with the observations of various authors that the final phase of thrusting towards the european platform became younger to the east along the west carpathian arc in addition palaeostress trajectory patterns for various periods are similar to those modelled by a rigid block indenting a viscous half space an attempt is made to complete the mosaic of information on the cenozoic evolution of the alpine carpathian pannonian area which in the models given by ratschbacher royden horvath bergerat and others is based on extrapolated data the object is to detail the timing and complexity of the local time and plate position related mechanisms in the context of the entire orogen in order to update previous rather simplified models

building and exhumation of the western carpathians new constraints from sequentially restored balanced cross sections integrated with low temperature thermochronometry western carpathian orogeny has been the subject of intense scientific debate due to the occurrence of enigmatic features leading several authors to provide contrasting geological models in this paper a new interpretation for the tectonic evolution of the western carpathians is provided based on i an analysis of the stratigraphy of the mesozoic tertiary successions across the thrust belt domains ii a reappraisal of the stratigraphy and sedimentology of the tectonic melange i e the so called pieniny klippen belt marking the suture between the inner and outer carpathians and iii the construction of a series of balanced and restored cross sections validated by 2d forward modeling our analysis provides a robust correlation of the stratigraphy from the outer to the inner carpathians independently of the occurrence of oceanic lithosphere in the area and allows for the reinterpretation of the tectonic relationships among the inner carpathians the outer carpathians and the pieniny klippen belt and the exhumation mechanisms affecting this orogenic belt in order to constrain the evolution during the last 20 ma our model also integrates previously published and new apatite fission track and apatite u th sm he data these latter indicate a middle late miocene exhumation of the pieniny klippen belt in this study the recent regional uplift of the pieniny klippen belt is described for the first time using a 2d kinematic model for the tectonic evolution of the western carpathians

continuity and episodicity in the early alpine tectonic evolution of the western carpathians how large scale processes are expressed by the orogenic architecture and rock record data

derivation of a1 type granites by partial melting of newly underplated rocks related with the tarim mantle plume the granitic rocks of the tarim large igneous province lip are temporally and spatially related to mafic intrusions however their tectonic setting and genetic relationship are debated here we report geochemical and zircon u pb hf isotopic results for three alkali feldspar granitic plutons in the halajun area western margin of the tarim block zircon u pb ages suggest these plutons were emplaced at 268 275 ma coeval with the neighbouring mafic ultramafic complexes and syenitic rocks these granitic rocks have high contents of sio 2 alkalis rb th zr and ree except eu and high ratios of feo mgo and ga al and show strong depletions in ba sr eu which are commonly observed in the a1 type granites zircon hf isotopes reveal a limited range of e hf t values from 1 0 to 3 5 for different samples from three granitic plutons obviously higher than those mostly

onshore offshore correlation of pleistocene rhyolitic eruptions from new zealand implications for tvz eruptive history and paleoenvironmental construction abstract taupo volcanic zone tvz in the north island new zealand is arguably the most active quaternary rhyolitic system in the world numerous and widespread rhyolitic tephra layers sourced from the tvz form valuable chronostratigraphic markers in onshore and offshore sedimentary sequences in deep sea cores from ocean drilling program odp leg 181 sites 1125 1124 1123 and 1122 located east of new zealand ca 100 tephra beds are recognised post dating the plio pleistocene boundary at 1 81 ma these tephras have been dated by a combination of magnetostratigraphy orbitally tuned stable isotope data and isothermal plateau fission track ages the widespread occurrence of ash offshore to the east of new zealand is favoured by the small size of new zealand the explosivity of the mainly plinian and ignimbritic eruptions and the prevailing westerly wind field although some tephras can be directly attributed to known tvz eruptions there are many more tephras represented within odp cores that have yet to be recognised in near source on land sequences this is due to proximal source area erosion and or deep burial as well as the adverse effect of vapour phase alteration and devitrification within near source welded ignimbrites despite these difficulties a number of key deep sea tephras can be reliably correlated to equivalent aged tephra exposed in uplifted marine back arc successions of wanganui basin where an excellent chronology has been developed based on magnetostratigraphy orbitally calibrated sedimentary cycles and isothermal plateau fission track ages on tephra significant pleistocene tephra markers include the kawakawa omataroa rangitawa onepuhi kaukatea kidnappers b potaka unit d ahuroa ongatiti rewa sub rewa pakihikura ototoka and table flat tephras six other tephra layers are correlated between odp core sites but have yet to be recognised within onshore records the identification of pleistocene tvz sourced tephras within the odp record and their correlation to wanganui basin and other onshore sites is a significant advance as it provides 1 an even more detailed history of the tvz than can be currently achieved from the near source record 2 a high resolution tephrochronologic framework for future onshore offshore paleoenvironmental reconstructions and 3 well dated tephra beds correlated from the offshore odp sites with astronomically tuned timescales provide an opportunity to critically evaluate the chronostratigraphic framework for onshore plio pleistocene sedimentary sequences e g wanganui basin cf naish et al 1998 quaternary science reviews 17 695 710

palaeomagnetic results from the fold and thrust belt of the western carpathians an overview the western carpathians are separated into an outer and inner carpathians both comprising several nappe systems by the extremely narrow and highly deformed pieniny klippen belt the main phase of deformation and thrusting took place during the late cretaceous in the inner carpathians at the end of cretaceous paleocene in the pieniny klippen belt and in the miocene in the outer carpathians in this paper a large amount of palaeomagnetic results of different quality available from several nappe stacks and from overstep sequences were reviewed and interpreted in terms of tectonics the data suggest that all three main units participated in two phases of anticlockwise rotation starting at 18 5 ma that is the outer carpathian nappes in front of the already consolidated alpine carpathian pannonian block became accreted to the block late cretaceous nappe transport neogene uplift of core mountains and possibly oroclinal bending of pre oligocene age can account for important differences in pre cenozoic palaeomagnetic declinations most of them exhibit less or no anticlockwise rotation suggested by the overstep sequences implying pre cenozoic clockwise rotations of variable angles

hannuoba intermediate mafic granulite xenoliths revisited assessment of a mesozoic underplating model abstract the intermediate mafic granulite xenoliths from tertiary hannuoba alkali basalts at the northern margin of the north china craton have been commonly considered to be formed by mesozoic basaltic underplating due to their abundant mesozoic zircons and compositionally higher mg molecular mg mg fe than rocks of the adjourning archean granulite terrain however such a mesozoic underplating model cannot account for the presence of precambrian zircons in the xenoliths and their evolved sr nd isotopic compositions which are dissimilar to expected basaltic melts from both the lithospheric and asthenospheric mantle instead their sr nd isotopic compositions resemble those of the granulite terrain and of the mesozoic low mg granitoids here newly acquired data and published data are combined to assess the mesozoic basaltic underplating model and to discuss the origin of the hannuoba intermediate mafic granulite xenoliths in terms of their relationships with the mesozoic magmatism and the granulite terrain zircons from a mafic granulite xenolith jn0724 which has geochemical and sr nd isotopic compositions indistinguishable from other mafic granulite xenoliths yield 2 5 ga u pb ages providing unambiguous evidence that the mafic xenoliths represent portions of ancient lower crust combined data reveals that the pyroxene rich granulite xenoliths can be best explained as restites left after partial melting of the ancient lower crust to produce the mesozoic low mg granitoids hf isotope analysis on mesozoic zircons from the hannuoba feldspar rich granulite xenoliths demonstrates that the zircons crystallized from mesozoic granitic magmas that were derived from ancient lower crust rather than from mesozoic underplated magmas the proposed scenarios successfully explain the age and compositional differences between granulite xenoliths and the granulite terrain and also provide new insights into derivation and evolution of granitic magmas it is concluded that most of the hannuoba intermediate mafic granulite xenoliths and the mesozoic granitoids reflect reworking of ancient lower crust rather than juvenile crust addition it provides strong evidence that crustal anatexis may be one of the major processes controlling the chemical differentiation of the continental crust our data imply that whether younger zircon ages in granulite xenoliths from many other archean cratons register post archean basaltic underplating could be tested by integrated analysis of u pb age and hf isotopes

petrogenesis of the dengzhazi a type pluton from the taihang yanshan mesozoic orogenic belts north china craton abstract the voluminous mesozoic monzonitic to monzogranitic rocks in the north china craton ncc mostly show high k calc alkaline and i type granitoids features the dengzhazi granitic pluton however shows features typical of a type granites the a type pluton was emplaced in the taihang yanshan orogenic belts of the northern margin of the ncc with zircon u pb ages of around 140 ma the dengzhazi a type granites are characterized by high sio 2 70 2 77 7 wt k 2 o na 2 o zr nb ga zn and y contents as well as high ga al ratios and extremely low cao ba sr in addition they show high zircon saturation temperatures 870 950 c low water and low oxygen fugacity all these features are consistent with the a type affinity of the pluton in situ hf isotopic analyses for the dated zircons show relatively small range of e hf t 13 to 17 they also have homogeneous initial nd isotopic compositions with e nd t ranging from 15 1 to 16 3 the hf and nd isotopic data suggest that the dengzhazi a type granites originated from a homogeneous crustal source probably the archean mafic intermediate granulites taking into account the high temperatures the low h 2 o and f o 2 of the magma system we believe that partial melting of the granulites should have been triggered by underplating of mantle derived magmas at the base of the mafic lower crust in an extensional regime the dengzhazi a type granite is the oldest pluton of the taihang yanshan mesozoic magma belts signifying the commencement of extensive underplating of mafic magmas and thus of lithospheric thinning in the northern ncc

sm nd and shrimp u pb zircon geochronology of high pressure granulites in the sanggan area north china craton timing of paleoproterozoic continental collision abstract high pressure hp mafic granulites are widely distributed as sheet like and lenticular bodies within highly deformed tonalitic gneisses in the sanggan area north china craton they have clockwise isothermal decompression and isobaric cooling p t paths implying continental collision tectonics whole rock analyses of 15 samples define a sm nd isochron yielding an age of 2547 87 ma with e nd t 2 7 corresponding nd depleted mantle model ages show a similar range and are interpreted as the age of the magmatic protolith to the high pressure granulites a combined sm nd mineral whole rock isochron from one of these samples yields an age of 1813 23 ma taken to represent the metamorphic age at an early stage on the cooling path shrimp u pb zircon analyses of three hp mafic granulite samples give uniform weighted mean 207 pb 206 pb ages of 1817 12 1817 12 and 1819 16 ma these ages are interpreted to date metamorphism of the hp granulites and with the sm nd mineral whole rock result strongly suggest that the time of continental collision was at ca 1 82 ga an additional sample records two age populations one at 1803 9 ma and an older one at 1872 16 ma the latter may indicate the timing of either an earlier metamorphic event or possibly the earliest stage of a single prolonged metamorphic episode that peaked at 1 82 ga these results confirm the paleoproterozoic age for assembly of the north china craton

age and composition of granulite and pyroxenite xenoliths in hannuoba basalts reflect paleogene underplating beneath the north china craton abstract in situ u pb and hf isotope systematics of zircons mineral chemistry of silicates and whole rock major element trace element and platinum group element pge data are reported for five mafic xenoliths granulites and pyroxenite in the hannuoba cenozoic basalts north china temperature estimates of 859 886 c suggest that these xenoliths are derived from the lower crust the whole rock compositions of the xenoliths show weakly negative eu anomalies eu 0 67 0 95 high sio 2 al 2 o 3 5 80 13 8 low ir 0 10 0 29 ppb and wide ranges of ni 4 5 235 ppm and la nb 1 70 7 64 most zircons in these samples are structureless and give paleogene 44 5 47 3 ma u pb ages minor populations have early mesozoic 210 220 ma late mesozoic 90 ma and neogene 14 ma ages the fractionation of pyroxene and plagioclase controlled the petrogenesis of the xenoliths which are interpreted as the products of basaltic underplating and fractionation cumulates in paleogene time 45 47 ma with assimilation of some older intermediate crustal components the paleogene underplating corresponds in time to lithosphere scale extension in the north china craton leading to the widespread formation of sedimentary basins most paleogene zircons have positive e hf up to 13 2 with uniform t dm 0 46 0 50 ga whereas most mesozoic zircons have negative e hf this evolution suggests a temporal decrease in the degree of crustal assimilation during the extension of the lithosphere in addition to complex modification in early and late mesozoic times the thermal event related to the eruption of the host magmas ca 14 ma also reheated the lower crust beneath the north china craton

u pb zircon and rutile ages from granulite xenoliths in the slave province evidence for mafic magmatism in the lower crust coincident with proterozoic dike swarms zircon and rutile in two garnet bearing mafic granulite xenoliths from a kimberlite in the archean slave province record proterozoic ages similar to the 1 27 ga mackenzie dike swarm or similar to the ca 2 23 2 21 ga malley mckay diabase dike swarms the xenolith sample site currently overlies thick lithosphere more than 700 km to the southeast of the proposed mackenzie plume center because lower crust was effectively shielded from the thermal effects of an upwelling plume by thick lithospheric mantle heat was most likely advected into the lower crust by lateral intrusion of mafic sills over distances of hundreds of kilometres from magma generation zones nearer the plume center

isotopic evidence from the ivrea zone for a hybrid lower crust formed by magmatic underplating the large mafic complex overlying mantle peridotites in the ivrea zone of northern italy originated by underplating of mantle derived magma onto the base of continental crust and may be a unique outcrop of typical mafic lower crust sampled elsewhere only by mafic xenoliths neodymium and strontium isotope data show that the magmas assimilated large and roughly uniform proportions of crustal material the resulting hybrid magmas have the same isotopic signature as and may represent the lower crustal roots of upper crustal hercynian granitoid intrusions

the evolution of the meliata hallstatt ocean and its significance for the early evolution of the eastern alps and western carpathians abstract the evolution of the meliata hallstatt ocean and of its southern and northern margin in the inner western carpathians are described the main rifting began during the pelsonian and the sea floor spreading ended at the beginning of the middle carnian contemporaneously with the raibl event the subduction began in the latest triassic and the final closing of the ocean accompanied by uplift of the adjacent marginal areas was dated as basal oxfordian remnants of the oceanic suboceanic sequence occur in two tectonic positions 1 obducted nappes that contain mostly as tectonic melanges the whole middle triassic to middle jurassic sequence including large bodies of dismembered ophiolites of ladinian to early carnian age 2 evaporite melanges at the base of nappes that are derived from the northern margin of the ocean these evaporite melanges consist of late permian evaporitic matrix and blocks of a dismembered ladinian to lower carnian ophiolitic sequence in the inner western carpathians both the northern and southern marginal sequences of the meliata hallstatt ocean and the transitions into the carbonate platforms are present in different nappes the southern marginal development is characterized by middle carnian distal clastic raibl beds and by middle jurassic rhyolithic volcanism both missing in the northern marginal development because the subduction related middle jurassic volcanism contemporaneous with the turbidites in the southern meliaticum is restricted to the marginal area south of the meliata hallstatt ocean southward directed subduction is indicated to the south on the adjacent carbonate platform a jurassic basin with aalenian to bajocian ophiolites opened this may be a back arc basin in the eastern alps only the northern marginal zone and the adjacent carbonate platform are preserved but parts of the dismembered ophiolites in the haselgebirge may be of ladinian to late carnian age like in evaporitic melanges in the same tectonic position in the inner western carpathians the final closing of the meliata hallstatt ocean is indicated by the abrupt end of the turbiditic sedimentntation in the oceanic suboceanic domain and by uplift in the marginal areas where lower oxfordian radiolarites are overlain by shallow water upper oxfordian to tithonian or neocomian limestones silica nappe hallstatt nappes with the evidence of the final closing of the meliata hallstatt ocean near the middle late jurassic boundary the cimmerian orogenesis in the genetic sense of engor 1984 1985 is now also proven in the inner western carpathians and in the eastern alps a continuation of the meliata hallstatt ocean into the pontide paleotethys cimmerian ocean sensu kozur 1990 through the transylbanian oceanic domain and the strandzha unit is assumed a connecction with the vardar ocean is impossible because in the vardar zone no triassic ophiolites or basic volcanics are present and the ophiolites have jurassic age

dating the lower crust by ion microprobe ion microprobe uthpb ages of zircons from granulite facies lower crustal xenoliths from north queensland australia correlate well with the ages of major orogenic episodes manifest at the earth s surface about half of the xenoliths contain proterozoic zircons which are similar in age to the episodes of high grade metamorphism of the older surface rocks all the xenoliths contain late paleozoic zircons which show a real 100 ma range in206pb238 u ages from 320 to 220 ma which is attributed to granulite facies metamorphism followed by slow cooling in the deep crust the paleozoic zircon ages coincide in time with the prolonged episode of eruption of voluminous felsic ash flows and intrusion of high level granites in this region 320 270 ma mineral and melt inclusions in the zircons provide clues to the origin of some of the xenoliths and coupled with the age information can be used to infer the geological processes operating in the lower crust the zircons from two mafic xenoliths contain felsic and intermediate melt inclusions implying at least a two stage history for these rocks involving either partial melting of a more felsic protolith or crystal accumulation from an evolved melt some of the zircons from the felsic xenoliths contain co2 rich fluid inclusions indicating that those zircons grew during high grade metamorphism the isotopic and chemical data for the whole rock xenoliths show that they originate from a segment of the lower crust which is a heterogeneous mixture of supracrustal and mafic mantle derived lithologies the major orogenic event responsible for the formation of that crust occurred in the late paleozoic when proterozoic supracrustal rocks were emplaced into the lower crust possibly along thin skinned thrust slices this was accompanied by intrusion of high temperature mantle derived melts which caused partial melting of pre existing crust the most likely setting for such tectonism is a continental margin subduction zone

deep crustal structure beneath the central and inner west carpathians abstract the 150 km long deep seismic transect 2t crossing the central part of the slovakian west carpathians has revealed an important whole crustal seismic suture zone dipping to the south this zone is dominated by six large and several minor reflection events because of our good knowledge of the surface geology and a large number of comparative studies of the eastern alps it is clear that the upper reflections are due to overthrust sheets that originated during compression of the apulian passive margin tectonically deeper reflections are interpreted as being due to ligurian oceanic igneous and sedimentary rocks sheared in ductile domain during continental collision with the brianconnais passive margin and continental masses north of the central apulian west carpathians cwc behind a significant strike slip fault are the cenozoic structures of the pieniny klippen belt and the magura rhenodanubian flysch south of the cwc the inner tethyan west carpathians iwc are thrust over the cwc the iwc originated during the late jurassic early cretaceous closure of the tethyan ocean and the seismic image of the iwc is different from the highly reflective cwc suture these three pre neogene nappe systems amalgamated as one overthrust system during the neogene subduction of the moldavian sea as the moldavian accretionary wedge the vepor gemer block of the west carpathians where the cretaceous suture is observed has been untouched by any major cenozoic deformation and has remained relatively rigid this unique structure therefore enables us to observe seismically the cretaceous suture zone which elsewhere in the european alpine belt has been almost completely destroyed more general data about the beginning of subduction and subsequent collision processes are presented significant compression and partial subduction of the apulian upper plate passive margin lasted from the hauterivian to the beginning of the turonian for about 25 30 ma subduction of the ligurian ocean occurred from turonian to campanian times after the early campanian the collision of the apulian and brianconnais plates took place

proterozoic zircon growth in archean lower crustal xenoliths southern superior craton a consequence of matachewan ocean opening granulite grade anorthositic and mafic xenoliths recovered from a jurassic kimberlite pipe near kirkland lake ontario are fragments of the lower crust that underlies the ca 2 7 ga abitibi greenstone belt of the superior craton cathodoluminescence imaging and or backscatter electron microscopy of zircon from four individual xenoliths reveals a complex crystallization history characterized by two main stages of zircon growth the age of the two stages has been constrained by combining imaging results with isotope dilution u pb dating of grain fragments and single grains minimum ages for the first crystallization stage in individual xeno liths are 2584 7 ma 2629 8 ma 2633 3 ma whereas an approximate crystallization age for a fourth sample is 2788 57 ma the second main stage of growth consists of chemically and isotopically distinct metamorphic zircon overgrowths times of solid state zircon growth are most broadly constrained in three samples to the interval between 2 52 ga to 2 40 ga and most precisely dated in a meta anorthosite at 2416 30 ma these complex zircons are intergrown with garnet and clinopyroxene of the host granulite facies assemblage and thus the paleoproterozoic ages of the metamorphic overgrowths are interpreted to reflect an interval of isobaric granulite grade metamorphism of the lower crust beneath the greenstone belt approximately 150 million years after craton formation this interval of metamorphism is broadly coeval with the intrusion of the matachewan dyke swarm across the southern superior craton and with mafic magmatism and deposition of huronian rift margin sediments 200 km to the south during the opening of the matachewan ocean it is proposed that a significant volume of magma intruded the crust mantle interface during rifting promoting isobaric metamorphism and zircon growth in the deep levels of the superior craton subsequent major rifting events along this margin apparently failed to produce a similar lower crustal response the results have important implications for the structure of lithosphere beneath archean continental crust

lower cretaceous carbonate platform facies western carpathians abstract two principal belts with urgonian carbonate platform sedimentation evolved during the barremian early albian in the western carpathians the northern grew on an accretionary belt in the convergence zone between the external and internal units the limestones here contain rich ophiolite detritus the other belt formed on the tatric ridge in both belts carbonate platforms were subsequently destroyed only slope sediments clastic fan sediments and calciturbidites are preserved in the studied successions

nd and sr isotopic compositions of lower crustal xenoliths from north queensland australia implications for nd model ages and crustal growth processes abstract sr and nd isotopic analyses of granulite facies lower crustal xenoliths from northern queensland australia fall on a mixing array on a 87 sr 86 sr vs 143 nd 144 nd diagram at 300 ma this was a time of intense orogeny in northern queensland attended by granitic magmatism in the upper crust and granulite facies metamorphism in the lower crust ion microprobe zircon dating indicates that most of the xenoliths formed at this time the sr and nd isotopic compositions of these rocks are therefore interpreted to have resulted from large scale mixing in the lower crust between mantle derived basaltic melts and isotopically evolved early proterozoic continental crust both end members are represented in the xenolith suite depleted mantle nd model age tdm ages for meta igneous granulite xenoliths having appropriate 147 sm 144 nd i e between 0 12 and 0 15 range from 1 9 to 2 2 ga for mafic melt like xenoliths and 1 2 to 1 4 ga for felsic melt like xenoliths however zircon ages on these same samples show that their protoliths crystallized during the permo triassic orogeny thus the old tdm ages for the mafic xenoliths do not accurately reflect their crustal residence ages possibly due to crustal assimilation or their derivation from an enriched mantle source this is a common feature of mafic granulite xenoliths the world over and highlights the difficulty in determining the age of mafic underplates hence the timing of crustal growth from tdm ages because the felsic melt like xenoliths lie on the mixing array their mid proteozoic tdm ages are likely to have resulted from mixing and therefore represent an average crustal residence age the combined data suggest that significant crustal growth and reworking occurs in the lower crust during phanerozoic crustal orogenies

nature and evolution of the lower crust in the eastern north china craton a review abstract in this paper published data for granulite terrain rocks exposed at the surface lower crustal xenoliths and mesozoic intermediate felsic igneous rocks from the eastern north china craton ncc are integrated to constrain the nature and evolution of the lower crust in this area u pb zircon dating shows that the protolith ages for most of the granulite terrain rocks are 2500 to 2600 ma and that many of them experienced 1800 1900 ma metamorphism lower crustal xenoliths entrained in volcanic rocks with ages varying from 460 to 10 ma suggest that the lower crust is dominated by neoarchean rocks although there may be minor rocks with ages of meso to paleoarchean 3000 ma 45 ma and possibly 1900 ma locally the mesozoic intrusive rocks although varying from diorite to granite and spanning from triassic to cretaceous contain 2500 ma inherited zircons and have magmatic zircons with hf crust model ages t dm hf c ages of 2500 2700 ma and whole rock sr nd isotopic compositions falling within the field of the granulite terrain rocks pointing to their derivation by the melting of neoarchean lower crust the combined data for the granulite terrain rocks lower crustal xenoliths and mesozoic intermediate felsic igneous rocks indicate that the present lower crust is dominated by rocks with neoarchean ages and is intermediate to mafic in composition i e sio 2 87 sr 86 sr i e nd t and e hf t of the lower crust at 130 ma are considered to be 0 705 to 0 716 10 to 28 and 13 to 28 respectively the e nd t range is very different from that proposed previously 32 to 44 the large range of e hf t for the lower crust implies that significant e hf t variations for magmatic zircons from the mesozoic intermediate felsic igneous rocks do not necessarily reflect mixing of mantle and crustal magmas as commonly thought instead they may reflect heterogeneity in the ancient lower crust given that the voluminous mesozoic intermediate to felsic igneous rocks in the eastern ncc are derived dominantly by partial melting of the archean lower crust it requires that a large amount of archean lower crust be restitic a restite origin can explain some of the hannuoba granulite xenoliths having higher mg than the granulite terrains it may be applicable to other parts of the world

zinc isotopic composition of the lower continental crust estimated from lower crustal xenoliths and granulite terrains abstract this study presents high precision zn stable isotope analyses for lower crustal rocks 9 granulites from archean terrains and 30 lower crustal xenoliths from the north margin of the north china craton ncc to understand the behavior of zn isotopes during deep crustal processes and the zn isotopic composition of the lower continental crust lcc the lower crustal xenoliths range in composition from mafic to felsic with mgo contents of between 0 4 to 20 2 wt the 66zn the permil deviation of the 66zn 64zn ratio from the jmc lyon standard values of lower crustal xenoliths range from 0 17 to 0 38 the lack of clear correlation of 66zn with geochemical indicators e g al2o3 cr ba la 87sr 86sr and 143nd 144nd indicates that assimilation of precambrian lower crust fluid metasomatism and accumulation of pyroxene and plagioclase had a limited effect on zn isotopic compositions of these lower crustal xenoliths the 66zn values 0 18 to 0 34 of garnet bearing mafic granulites decrease with increasing feo t and v contents which are likely results of the accumulation of fe ti oxides the average 66zn of the lower continental crust is estimated to be 0 29 0 02 95 se using lower crustal xenoliths from the neogene hannuoba basalts this 66zn value is similar to the estimated value 0 28 0 04 95 se obtained from the granulites from archean terrains suggesting that there is no significant difference in the zn isotopic composition between the archean and present day lower continental crust combining the 66zn data of the lower crustal xenoliths and granulite terrains and different weighting methods the zn isotopic composition of the lower continental crust is estimated to be 0 28 0 04 95 se

deep probe imaging the roots of western north america analysis of the lithoprobe deep probe and southern alberta refraction experiment data sets focusing on the region between deep probe shots 43 and 55 has resulted in a continental scale velocity structural model of the lithosphere of platformal western laurentia reaching depths of 150 km three major lithospheric blocks were investigated i the hearne province a typical continental archean cratonic province lying beneath the western canada sedimentary basin ii the wyoming province an even older block of phanerozoic modified archean crust with an enigmatic lower lithosphere and iii the yavapai mazatzal province proterozoic terranes underlying the colorado plateau and southern rocky mountains in this study the northern two of these regions are investigated with a modified ray theoretical traveltime inversion routine that respects the spherical geometry of the earth the resulting crustal velocity structure combined with supporting geological and geophysical data reveals that the medicine hat block mhb lying between the hearne and wyoming provinces is a third independent archean crustal block the subcrustal lithosphere along the profile is homogeneous in velocity structure but two significant northward dipping reflectors are apparent and interpreted as relic subduction zones associated with sutures between the three archean blocks the hearne crust is typical of an archean shield or platform both in its thickness of 34 50 km and its seismic velocity structure the crust of the archean mhb and wyoming province which ranges in thickness from 49 to 60 km includes a 10 30 km thick high velocity layer interpreted to be proterozoic in age such a feature is unexpected beneath archean crustal provinces but if the region is considered to be the remanent marginal portion of a larger archean continent then the interpreted proterozoic underplating and lack of an archean lithospheric root can be explained the variable topography along the reflective upper and lower boundaries of this layer especially within the mhb suggests considerable variability in its emplacement and subsequent tectonic history

2 7ga crust growth in the north china craton abstract numerous u pb zircon geochronological data show that rocks of the precambrian basement from the north china craton ncc have two age peaks at 1 8 1 9 and 2 5 ga respectively whether the most prominent peak at 2 5 ga really registered a period of major crustal growth in the ncc thus making it different from the global major crustal growth period of 2 7 ga is uncertain in this paper we carried out in situ u pb and hf isotopes of zircons and whole rock chemical and sr nd isotopic analyses for six precambrian samples from the ncc in order to understand crustal growth and reworking the samples have u pb zircon ages of 1 8 1 9 2 5 and 2 7 ga respectively the two samples with u pb zircon ages of 2 7 ga have zircon hf model ages t hf dm and or whole rock nd model ages t nd dm close to their u pb zircon ages it is clear that they represent juvenile crust generation thus providing direct evidence that 2 7 ga was a period of new crustal addition the samples with u pb zircon ages of 1 8 and 2 5 ga also have t hf dm and or t nd dm similar to those of the 2 7 ga samples indicating that these samples also have a mantle extraction time of 2 7 0 1 ga and their 1 8 and 2 5 ga ages may represent periods of reworking of the 2 7 0 1 ga crust rather than juvenile crust generation the reworking is manifested either as metamorphism or as lower crustal derived magmatism specifically the 2 5 ga tonalitic gneiss sample is thought to be derived by melting of the 2 7 ga lower crust since many 2 5 ga tonalite trondhjemite granodiorites ttgs and calc alkaline granitoids in the ncc contain 2 7 ga xenocrystic and inherited zircons we postulate that they are probably also derived mainly from the 2 7 0 1 ga crust our data imply that the quantity of 2 7 ga continental crust in the ncc is probably greater in extent than that exposed today

tracing magma mixing in granite genesis in situ u pb dating and hf isotope analysis of zircons in situ zircon u pb and hf isotopic data have been determined for mafic microgranular enclaves and host granitoids from the early cretaceous gudaoling batholith in the liaodong peninsula ne china in order to constrain the sources and petrogenesis of granites the zircon u pb age of the enclaves 120 1 ma is identical to that of the host monzogranite 120 1 ma establishing that the mafic and felsic magmas were coeval the hf isotopic composition of the enclaves ehf t 4 5 to 6 2 is distinct from the host monzogranite ehf t 15 1 to 25 4 indicating that both depleted mantle and crustal sources contributed to their origin the depleted mantle component was not previously revealed by geochemical and nd and sr isotopic studies showing that zircon hf isotopic data can be a powerful geochemical tracer with the potential to provide unique petrogenetic information some wall rock contamination is indicated by inherited zircons with considerably older u pb ages and low initial hf isotopic compositions hafnium isotopic variations in early cretaceous zircons rule out simple crystal liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks instead mixing of mantle derived mafic magmas with crustal derived felsic magmas coupled with assimilation of wall rocks is compatible with the data

magma mixing in the kalaqin core complex northern north china craton linking deep lithospheric destruction and shallow extension abstract the widespread mesozoic magmatism in the north china craton ncc has received considerable attention as a trigger for large scale lithospheric destruction here we investigate the early cretaceous jiguanzi adamellite from the northern part of the ncc which is contemporaneous with shallow extensional deformation and deep lithospheric destruction this intrusion emplaced at ca 133 ma is located in the foot wall of the kalaqin metamorphic core complex mcc and occurs as a synextensional ring complex with numerous magmatic equigranular group 1 and porphyritic group 2 enclaves hornblende and plagioclase from the host adamellite and xenocrysts of group 2 enclaves show distinct inverse zoning with mg and ca rich mantle the group 2 enclaves are characterized by plagioclase xenocrysts hosting hornblende biotite and apatite inclusions quartz ocelli with fine grained rim enriched in biotite and hornblende and poikilitic biotite surrounded by hornblende geochemically the host intrusion is calc alkaline to alkaline and metaluminous with variable contents of sio 2 60 70 72 20 wt al 2 o 3 14 19 17 22 wt na 2 o k 2 o 6 16 9 42 wt and mg values 28 0 47 7 whereas the group 2 enclaves exhibit low sio 2 54 05 55 55 wt high fe 2 o 3 8 18 8 64 wt and tio 2 2 08 2 28 wt and moderate mg 44 0 44 1 both the host intrusion and group 2 enclaves are enriched in large ion lithophile and light rare earth elements and depleted in high field strength elements and heavy rare earth elements except that the latter has lower ba and high nb ta and ti contents the major and trace element contents of the group 1 enclaves are broadly similar to those of the host intrusion analyses of sr nd hf isotopes in the host intrusion and in group 1 and group 2 enclaves show 87 sr 86 sr i host 0 70600 0 70618 e nd t host 8 2 to 9 6 t 2dm nd host 1592 1706 ma e hf t host 9 2 to 12 0 87 sr 86 sr i group 1 0 70590 0 70635 e nd t group 1 9 6 to 10 4 t 2dm nd group 1 1705 1775 ma 87 sr 86 sr i group 2 0 70586 0 70587 e nd t group 2 0 6 to 0 7 t 2dm nd group 2 971 978 ma and e hf t group 2 4 5 to 10 0 the mineralogical and geochemical features suggest that the host jiguanzi adamellite and group 2 enclaves were generated through synchronous mixing and fractional crystallization accompanied by mechanical and chemical exchanges the group 1 enclaves showing cogenetic affinity with their felsic host represent the mechanical concentrations of mafic minerals and accessory phases from evolved hybrid host magma magma sources of the host jiguanzi adamellite and its group 1 enclaves are dominated by lower crust lcc components whereas those for the group 2 enclaves are dominated by metasomatized lithospheric mantle the magma genesis involved complex multi stage crust mantle interaction processes including 1 lithospheric mantle modification induced by triassic subduction of the paleo asian ocean slab and or triassic cretaceous asthenospheric melt underplating and 2 melting of the ancient lcc and lithospheric mantle and hybridization of mantle and crust derived melts in conjunction with regional geological and geochemical data we argue that the late jurassic early cretaceous shallow extension events represented by the kalaqin mcc triggered deep seated multi stage magmatism and lithospheric destruction and the continuous generation of magma further strengthened the extension and result in more extensive lithospheric thinning

age diversity of the deep crust in northern mexico zircons extracted from five chemically diverse granulite facies crustal xenoliths from la olivina cinder cone chihuahua mexico were analyzed by shrimp ion microprobe in order to determine the age of the deep crust in northern mexico the results demonstrate the extreme age diversity of the deep crust in this region zircon populations of ca 1 25 37 ca 200 350 1100 and 1370 ma are present nearly all samples contain 0 20 ma zircons reflecting recent granulite facies conditions in the deep crust in addition high grade metamorphism may also have occurred at 200 ma and 1 10 ga grenville the paucity of carboniferous permian ages in the xenoliths suggests that the ouachita collision event provided little thermal input to the deep crust beneath la olivina

petrology and geochemistry of the shuiquangou syenitic complex northern margin of the north china craton the palaeozoic shuiquangou complex is a major syenitic intrusion along the northern margin of the north china craton that hosts a number of gold deposits despite the heterogeneity of the complex in terms of texture and modal mineralogy mineral compositions whole rock chemistry as well as radiogenic isotope compositions show a well constrained magmatic evolutionary trend the whole rock major element and ree trends are broadly consistent with fractional crystallization dominated by clinopyroxene amphibole and accessory minerals initial nd sr isotopic compositions suggest that the parent magmas to the complex were ultimately derived from an enriched mantle with relatively high initial 87 sr 86 sr ratios and negative nd values the enrichment of the mantle source was probably inherited from an older lithospheric mantle beneath the north china craton sr isotopic data indicate that the rb sr system was disturbed by later alteration processes whereas nd isotopes are essentially unaffected and exhibit a broad trend consistent with simultaneous fractional crystallization and country rock assimilation afc quantitative modelling indicates that a very high bulk distribution coefficient for nd and low ratios of assimilation to crystallization are needed to produce the observed nd isotope evolutionary trend

palaeotectonic reconstruction of the central alpine mediterranean belt for the neogene abstract the structural patterns of the central alpine mediterranean belt ranging from the eastern balearic sea to the western aegean sea have been reconstructed for 10 and 20 ma b p they have been obtained in the frame of the africa europe convergence through a step by step rigid plastic reconstruction using available geological and palaeomagnetic data all domains outlined in the present situation are shown in the palaeotectonic reconstructions while the structural pattern at 10 ma b p is similar to that of the present except for the tyrrhenian region that at 20 ma b p differs from it in most areas the well known adriatic promontory of the african plate is recognizable in both pictures and at 20 ma its original boundaries are revealed as strongly deformed flysch belts the adriatic promontory was broken up in the early or middle miocene into two microplates of comparable sizes the norther one is the adriatic microplate s s which suffered counter clockwise rotation by 20 relative to africa around the alpine apenninic whirl centre the southern one is the apulian microplate first outlined in the present work including the south apenninic and the apulian s s south of the gargano peninsula platforms which suffered no rotations relative to africa but which moved towards the northeast of it the primary reason for the break up of the adriatic promontory was most probably the opening of the balearic ligurian sea since 10 ma the adriatic and apulian microplates were again integral with africa or perhaps they moved towards the west relative to africa pushed by the turkish microplate the orogeny in the area discussed is mostly linked with collisional processes on the boundaries of the adriatic apulian unit

new paleomagnetic results from the upper cretaceous red marls of the pieniny klippen belt western carpathians evidence for general ccw rotation and implications for the origin of the structural arc formation abstract the pieniny klippen belt pkb is a narrow arcuate structure separating the central and outer western carpathians formed during several cenozoic deformational stages the primary aim of this study was to obtain paleomagnetic constraints for the mechanism of formation of the arc we investigate albian santonian red pelagic marls from 14 localities distributed along a strike length of ca 400 km ams measurements reveal a pattern characteristic of weakly deformed sedimentary rocks and magnetic lineations do not correlate with the general strike of the pkb paleomagnetic analysis revealed well defined hematite based ancient magnetization components at 13 localities which are dated using fold and inclination tests a within locality fold test is negative for two localities exhibiting large ccw rotations of similar magnitude situated at the two ends of the pkb remanences of pre folding age were documented for 11 localities with an overall mean paleomagnetic direction of d 311 i 53 and 95 11 the indicated general ccw rotation most probably took place during the miocene together with western central and outer carpathians paleolatitudes for the pkb indicate a considerable separation from the southern margin of stable europe leaving space for coordinated rotation a paleomagnetic oroclinal test involving all localities with primary magnetizations was negative when localities with monoclinal steep dips are omitted due to possible declination bias the overall mean paleomagentic direction does not change significantly but correlation is observed between the general trend of the pkb and the paleomagnetic declinations thus we conclude that the present shape of the arc can be partly due to oroclinal bending this must have happened before oligocene since paleomagnetic declinations for neighboring paleogene basins in the central and outer western carpathians reveal a uniform ccw rotation of ca 50 magnitude irrespective of the position of the localities in relation to the carpathian arc

subducted continental margin imaged i n the carpathians of czechoslovakia deep seismic reflection data across the frontal part of the carpathian arc image subducted european continental crust beneath the thick accretionary wedge of the neogene west carpathian arc the upper continental crust of the lower plate is depressed with high curvature to a depth off about 25 km below the arc but the corresponding moho appears to be nearly horizontal flow within the lower crust may account for strain mismatch between the two parts of the subductiog continent although the subdued relief on the moho may partly have evolved after subduction ceased perhaps owing to a gabbro eclogite phase change in a mafic lower crust

proterozoic zircon ages from lower crustal granulite xenoliths kola peninsula russia evidence for crustal growth and reworking lower crustal garnet granulite xenoliths from beneath the kola karelian domain of the fennoscandian shield russia were brought to the surface in devonian lamprophyre diatremes to establish the relationships between processes in the lower crust and geological events recorded in the upper crust zircons from two of these xenoliths were dated by ion microprobe one xenolith contains mainly primary magmatic zircon grains that yielded an average age of 2 47 ga the kola karelia domain formed part of a large basaltic igneous province in the earliest proterozoic c 2 45 ga thus such xenoliths probably represent deep seated equivalents of this magmatism a second garnet granulite xenolith contained only internally featureless zircons most of these yielded concordant ages between 1 77 1 61 ga this age distribution indicates that at c 1 7 ga the protolith was migmatized during an event that may be related to emplacement of granites and pegmatites in the region however two zircon grains in the second xenolith gave much younger ages of 1 47 and 1 45 ga this spread in the ages is related to an unknown younger event around 1 4 ga or perhaps later when the c 1 7 ga migmatite zircons were reset to variable degrees these zircon ages are evidence for the complex evolution of the lower crust recording multiple events some of which are not currently expressed on the surface

pre oceanic rifting in the alps abstract a review of mesozoic mainly earliest jurassic to mid jurassic rifting in the western and central alps normal faults with throws up to a few kilometers are moderately inclined and involve rotation of crustal blocks the amount of stretching and thinning of the crust by jurassic rifting is discussed the rifting 200 165 m y pre dates the opening of the alpine oceans by spreading and transform movements 165 110 m y

40ar 39ar dating of the inner carpathians variscan basement and alpine mylonitic overprinting abstract the 40ar 39ar dating method has been applied to mylonitic rocks from the carpathian mountains these are variscan metamorphic and magmatic rocks that were overprinted during the alpine orogeny the inner western carpathians are conventionally described as a nappe pile formed by northward movement and then subjected to intense strike slip and normal faulting resulting in the formation of mylonitic rocks precise stratigraphie ages provide clear constraints for the geodynamic interpretation related to the succession of alpine events such as the pre gosauian phase during the turonian 90 ma the 40ar 39ar method allows to define the age of the ancient variscan basement and the ages of the alpine overprinting in the high and low tatras mineral ages of around 340 350 ma are yielded by gneisses micaschists and amphibolites providing a younger limit for the variscan metamorphism the emplacement cooling of the variscan granites is defined between 305 and 335 ma these rocks are overprinted by several alpine tectonothermal events the major events are dated at 65 and 86 88 ma during which the mylonites were generated an older event around 140 ma earlier defined by the rb sr method is now confirmed by the 40ar 39ar method dating of phengites extracted from high pressure metamorphic triassic rocks from the kosice area yields new data for the age of this event at 155 ma