Hornblende Ages Research Articles

K-Ar geochronology of the western half of Tupungato volcano: the 1984 Hildreth-Drake-Fierstein expedition

During 23 days in February 1984, the team made up by Wes Hildreth, Bob Drake, and Judy Fierstein explored and sampled much of the western half of the Tupungato volcano, a Chile-Argentina natural landmark at ~33.3° S, including its summit area. The project remained unfinished, and this impressive, ~6,550 m-high volcanic edifice was no longer the focus of significant studies. Forty years later, Wes Hildreth has given up and graciously handed over to us all his data, so it can now be shared with the geological community. Bob Drake dated six samples of Tupungato through the K-Ar technique. In a dacite dome from the southern summit, interpreted as the youngest Tupungato activity, a hornblende age of 831±116 ka is reported. A long-standing controversy is thus resolved: Tupungato has no Holocene activity. 1.26±0.6 Ma to 932±90 ka plagioclase ages are reported from three andesite lavas from the lower to middle flank of the volcano, constraining the age of the edifice to the Early Pleistocene. Finally, hornblendes from two pre-Tupungato dacitic deposits at the volcano’s western foothill were dated at 11.4±0.5 Ma and 9.45±0.6 Ma. These much older deposits may represent pyroclastic rich facies similar to the ones described in the upper sections of the Tunuyán Conglomerates, in Argentina.

Late Oligocene to Early Pliocene Exhumation and Structural Development in the Western Himalaya, Northern Pakistan: Implications for the Cenozoic Metamorphic Overprint

Abstract New middle Miocene to Pliocene (~14–3 Ma) apatite fission track (AFT) cooling ages combined with published K–Ar/Ar–Ar and zircon fission track (ZFT) ages from the Hazara and Swat regions of Pakistan are used to explain the Oligocene to Pliocene structural evolution in the Western Himalaya. The structural model explains the distribution of K–Ar/Ar–Ar ages in three distinct age groups (Proterozoic, Paleozoic-Mesozoic, and Eocene to Oligocene). The Proterozoic to Mesozoic sequence of northern Hazara and Swat experienced elevated temperature and pressure conditions, evident by reset Eocene to Oligocene K–Ar/Ar–Ar hornblende and Eocene to Miocene muscovite ages, caused by Kohistan overthrusting the Indian margin during and after the India–Asia collision. Samples from the Indus syntaxis with Paleo to Mesoproterozoic K–Ar/Ar–Ar hornblende ages and Eocene to Oligocene Ar–Ar muscovite ages show no signs of Cenozoic metamorphism; these samples were thermally imprinted up to the Ar–Ar muscovite closure temperature. Neoproterozoic to Lower Paleozoic rocks from the southern parts of Hazara and Swat show Mesozoic to Oligocene partially reset Ar–Ar muscovite ages and preservation of Ordovician metamorphism. The combined analysis of published K–Ar/Ar–Ar (muscovite), ZFT, and new AFT ages (~14–12 Ma) suggests that the Main Central thrust/Panjal thrust was active from Oligocene to early Miocene (~30–18 Ma), and the Nathia-Gali and Main Boundary thrusts were active from the middle to late Miocene (~14–9 Ma) in the Hazara area. New and published AFT ages (~6–3 Ma) from the Indus syntaxis suggest that early Pliocene tectonic thickening in the hinterland formed the N–S trending Indus anticline, creating an erosional half window in the Main Mantle thrust, forming the Indus syntaxis, and dividing the Main Central thrust sheet into the Hazara and Swat segments.

Episodes of Early Pleistocene West Antarctic Ice Sheet Retreat Recorded by Iceberg Alley Sediments.

Ice loss in the Southern Hemisphere has been greatest over the past 30 years in West Antarctica. The high sensitivity of this region to climate change has motivated geologists to examine marine sedimentary records for evidence of past episodes of West Antarctic Ice Sheet (WAIS) instability. Sediments accumulating in the Scotia Sea are useful to examine for this purpose because they receive iceberg‐rafted debris (IBRD) sourced from the Pacific‐ and Atlantic‐facing sectors of West Antarctica. Here we report on the sedimentology and provenance of the oldest of three cm‐scale coarse‐grained layers recovered from this sea at International Ocean Discovery Program Site U1538. These layers are preserved in opal‐rich sediments deposited ∼1.2 Ma during a relatively warm regional climate. Our microCT‐based analysis of the layer's in‐situ fabric confirms its ice‐rafted origin. We further infer that it is the product of an intense but short‐lived episode of IBRD deposition. Based on the petrography of its sand fraction and the Phanerozoic 40Ar/39Ar ages of hornblende and mica it contains, we conclude that the IBRD it contains was likely sourced from the Weddell Sea and/or Amundsen Sea embayment(s) of West Antarctica. We attribute the high concentrations of IBRD in these layers to “dirty” icebergs calved from the WAIS following its retreat inland from its modern grounding line. These layers also sit at the top of a ∼366‐m thick Pliocene and early Pleistocene sequence that is much more dropstone‐rich than its overlying sediments. We speculate this fact may reflect that WAIS mass‐balance was highly dynamic during the ∼41‐kyr (inter)glacial world.

Tonian and Cryogenian 40Ar/39Ar hornblende and muscovite ages for the São Gabriel Terrane, Dom Feliciano Belt, southern Brazil

Geochronological 40Ar/39Ar investigations in the São Gabriel Terrane, Dom Feliciano Belt, Brazil, associated with previous structural geology and geochronology studies, allow for a better understanding of Neoproterozoic events in this region. The Sanga da Cachoeira Complex and metasedimentary rocks of Arroio Marmeleiro Complex (U–Pb provenance age > 1.5 Ga) show 40Ar/39Ar spectra with no successful plateau age, attributed to the alteration or excess 40Ar. Tonian (ca. 729 ± 9 Ma) 40Ar/39Ar hornblende age of metadiorites of the Cambaí Complex (U–Pb protolith age ca. 750 Ma) is interpreted to register the age of an early structure in this unit, where hornblende is a mineral of the composite banding fabric formed by remnants of original bedding, locally preserved, and added by metamorphic recrystallization. A cooling rate of at least 5°C/Ma is suggested for the Cambaí Complex, from its crystallization to its metamorphic event. The Cryogenian (<690 ± 8 Ma) 40Ar/39Ar hornblende and muscovite ages belonging to the Imbicuí Complex (protolith age ca. 869 Ma) whose ages are related to the late structure formed by a short and intense activity of the Cerro Branco Shear Zone, which experienced a rapid cooling of 12°C/Ma. The ages from the rocks belonging to such a diverse tectonic setting, including continental and intraoceanic island arc, oceanic crust low‐K amphibolites and meta‐harzburgites, and metasediments, presenting similar nature and fabric orientation, indicate that major tectonic‐magmatic–metamorphic activity was responsible for the accretion of these lithotectonic units that gave rise to the São Gabriel Terrane in southern Brazil.

Geochemistry and geochronology of the Jawornik granitoids, Orlica-Śnieżnik Dome, Sudetes, Poland

The Jawornik granitoids intrude, in vein-like form, a sequence of a polymetamorphic metavolcanic and metapelitic rocks of the Orlica-Śnieżnik Dome, Sudetes, Poland. This paper provides whole-rock geochemical data, sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon geochronological data as well as 40Ar-39Ar age determinations to constrain the genetic and temporal relationships of the different rock types forming these veins. Based on macroscopically visible features of the granitoids and their relationship with tectonic structures visible in the country rocks, four varieties of the Jawornik granitoids have been distinguished: amphibole- and biotite-bearing granites (HBG), biotite-bearing granites (BG), biotite- and muscovite-bearing granites (BMG) and muscovite-bearing granites (MG). The Jawornik granitoids as a whole show a limited but significant variation in major element chemical composition, with SiO2 ranging from 65 to 76 wt.% (average 69.16 wt.%, n = 24). They are subalkalic, peraluminous and calc-alkaline [average A/CNK = 1.07, average (Na2O + K2O) = 7.75, average (Fe2O3t/(Fe2O3t + MgO) = 0.59]. Close inspection of their geochemical parameters showed that the samples investigated can be subdivided into two groups. The first group, the HBG, BG, and BMG varieties, comprising most of the granitoids in the Złoty Stok-Skrzynka Tectonic Zone, were formed by melting of greywackes or/and amphibolites. The MG, belonging to the second group, were formed by partial melting of a more felsic source. The HBG yielded a zircon U-Pb age of 351 ±1.3 Ma and well-defined 40Ar-39Ar plateau ages for hornblende (351.1 ±3.9 Ma) and coexisting biotite (349.6 ±3.8 Ma), indicating probably the oldest magmatic event in this region. Zircons from the MG, the youngest rock variety on the basis of their relationship with the tectonic structures in the host rocks, yielded a U-Pb age of 336.3 ±2.4 Ma, though based on three points only. The biotites and muscovites from the BMG have 40Ar-39Ar plateau ages of 344.1 ±4.7 Ma and 344.6 ±3.8 Ma, respectively. These data, in combination with already published isotopic ages, suggest that the Jawornik granitoids intruded host rocks of the Orlica-Śnieżnik Dome in three stages, at ~350, ~344 and ~335 Ma.

Magma Loading in the Southern Coast Plutonic Complex, British Columbia and Washington

Abstract The southen end of the 1800 km long Coast Plutonic Complex (CPC), exposed in the Harrison Lake area of British Columbia and in the North Cascades of Washington, bears a record of great crustal thickening -20 to 40 km in localized zones during Late Cretaceous times. During this period, the CPC was positioned at the continental margin during collision/subduction of the Farallon plate. Arc magmatism and regional orogenic contraction were both active as potential crustal thickening processes. Magmatism is favored in this report as the dominant factor based on the delineation of four spatially and temporally separate loading events, the close association of the loaded areas with emplacement of large plutons, and a paucity of evidence of deep regional tectonic contraction. The timing and spatial location of crustal loading events are documented by the following: zircon ages in plutons; an early event of low pressure in pluton aureoles evidenced by andalusite, now pseudomorphed by high-pressure minerals; high pressures in country rock in pluton aureoles measured by mineral compositions in the assemblages garnet-biotite-muscovite-plagioclase and garnet-aluminum silicate-plagioclase; high pressures recorded in plutons by Al-in-hornblende barometry; and uplift ages of plutons derived from K-Ar and Ar-Ar ages of micas and hornblende in plutons.

Conditions and timing of incorporation of ophiolite into orogenic crust during oblique convergence, Central Anatolia

Ophiolitic fragments scattered over a wide area of Central Anatolia exhibit varying degrees of metamorphism, from unmetamorphosed to upper amphibolite facies, although geochemical similarities suggest they are all part of the Central Anatolian Ophiolite (CAO). Magmatic crystallization of oceanic crust in the CAO at ~ 91 Ma coincided with high-grade metamorphism of rocks that underlie the southern, highest grade part of the CAO, raising questions about the tectonic relationship of the ophiolite to underlying metasedimentary and plutonic rocks. New geochronology results show that the 40Ar/39Ar hornblende age of amphibolite-facies metagabbro in the high-grade metamorphic part of the CAO is ~ 87 Ma, similar to hornblende ages from amphibolite in the underlying Nigde metamorphic/plutonic massif. Biotite in a deformed quartzofeldspathic rock associated with high-grade meta-ophiolitic rocks yielded an 40Ar–39Ar age of ~ 78 Ma, similar to biotite ages from the Nigde Massif. Hornblende in gabbro from unmetamorphosed CAO yielded an older 40Ar/39Ar age of ~ 90 Ma, similar to the previously determined crystallization age of the ophiolite. These data indicate that the southern part of the ophiolite was incorporated into and therefore metamorphosed and deformed with the orogenic mid-crust now exposed in the Nigde metamorphic–plutonic complex, whereas the northern, unmetamorphosed part of the ophiolite was obducted onto the continent. This distinct difference in different parts of the ophiolite may indicate oblique collision or irregularities in the continental margin, resulting in part of the ophiolite being incorporated into the orogenic crust and subsequently exhumed and cooled with it, and another part being obducted.

Tectonic and Geodynamic Settings of Formation of the Mesozoic Cumulative Dunite–Wehrlite–Olivine Clinopyroxenite—Gabbro Massif in Eastern Chukotka

The dunite–wehrlite–clinopyroxenite–gabbro massif in Eastern Chukotka, a key object for geodynamic reconstructions of the Vel’may terrane, which represents one of the segments of the southern border of the Chukotka folded system (Chukotka microcontinent, or Arctic Alaska–Chukotka microplate), is investigated. Mineralogical and petrological–geochemical studies of rocks of this massif are carried out. A comparative analysis of the primary mineralogy and formation conditions of cumulative rocks of dunite–wehrlite–pyroxenite–gabbro assemblages from modern island-arc systems, mantle transition zones, and crustal sections of ophiolites and ancient island arcs shows that the rocks studied are cumulates crystallized from a tholeiitic melt in an intraoceanic island arc at a moderately high pressure. The 40Ar/39Ar age of magnesian hornblende from gabbro indicates the massif was formed no later than in the Early–Middle Jurassic. The petrological and geochemical modeling suggests that the analyzed olivine clinopyroxenites and gabbros are probable plutonic comagmates of the Late Triassic island-arc basalts and dolerites of the Vel’may terrane. The arc segment represented by these rocks of the Vel’may terrane was probably part of the system of island arcs which had been reconstructed in this region for the age interval of 163 to 230 Ma. In addition, there is a tendency for the rejuvenation of the Middle Triassic–Late Jurassic island-arc magmatism in the direction from west to east, namely, from the South Anyui terrane of Western Chukotka through the Vel’may terrane of Eastern Chukotka to the Angayucham terrain of Alaska.

Tectonic evolution of the Chinese Tianshan Orogen from subduction to arc-continent collision: Insight from polyphase deformation along the Gangou section, Central Asia

AbstractThe Central Asian Orogenic Belt, as the largest accretionary orogen on Earth, is an ideal candidate to study the geodynamics of convergent plate boundaries through a prolonged period. The evolution of this orogen has been explained by different tectonic models, which incorporated one, or a combination, of the following mechanisms: lateral stacking of arc systems along major shear zones, arc amalgamation, oroclinal bending, and trench migration. Here we elucidate major mechanisms responsible for the tectonic evolution of the Central Asian Orogenic Belt, focusing on the Chinese Tianshan Orogen in the southern Central Asian Orogenic Belt. Structural observations from the ∼50-km-long Gangou section show evidence of polyphase deformation. The earliest episode of orogen-parallel sinistral shearing, constrained to the Early Devonian (ca. 399 Ma) by syn-deformational intrusions, was possibly controlled by oblique subduction. This was followed by an episode of ∼NE–SW contractional deformation, dated at ca. 356 Ma (40Ar/39Ar age of syn-deformational hornblende), and likely associated with an episode of trench advance. The third stages of deformation during the latest Carboniferous and Permian involved ∼NE-SW contraction, orogen-parallel extension, and dextral transpression. Our new geochronological data constrain the timing of orogen-parallel extension to ca. 303–293 Ma, and confirm that dextral activation along shear zones occurred during the Permian. The results highlight the role of trench migration, oblique tectonics, and syn-collisional orogen-parallel extension in the build-up of the Central Asian Orogenic Belt, and contribute to the pre-collisional reconstruction of this orogenic system.

The Ediacarian-Cambrian uplift history of western Dronning Maud Land: New 40Ar-39Ar and Sr/Nd data from Sverdrupfjella and Kirwanveggan, the source of the Urfjell Group and tectonic evolution of Dronning Maud Land within the Kuunga Orogeny and Gondwana amalgamation

Biotite 40Ar/39Ar ages from Mesoproterozoic gneisses from Kirwanveggan and Sverdrupfjella vary between 868–497 Ma and 547–326 Ma respectively. Hornblende ages from Kirwanveggan and Sverdrupfjella gneisses vary between 1067–480 Ma and 550–450 Ma respectively. The similar ages for hornblende and biotite from Sverdrupfjella are consistent with rapid uplift in a single event. The differences in ages between biotite and hornblende from Kirwanveggan imply differential heating southwards through Kirwanveggan and suggest heating over northern Kirwanveggan at ~500 Ma with central to southern Kirwanveggan preserving older Mesoproterozoic 40Ar/39Ar ages in hornblende. The heating is inferred to result from burial in the footwall of a nappe.Biotite ages throughout Sverdrupfjella and Kirwanveggan are similar to ages of deposition of Urfjell Group clastic sediments, with the Urfjell Group defining the southern limit of uplift and deposition at ~530 Ma. Radiogenic Nd/Sr isotope characteristics of the Urfjell Group are similar to basement gneisses from nearby Kirwanveggan and eastern Sverdrupfjella, Gjelsvikfjella and central Dronning Maud Land (CDML) but dissimilar to western Sverdrupfjella. In contrast detrital zircon patterns of the Urfjell Group suggest derivation from a source similar to distant CDML, ca. 500 km away.Tectonic analysis of Dronning Maud Land (DML), using available and new Sr/Nd radiogenic isotope and 40Ar/39Ar data, SHRIMP U/Pb zircon data, P-T-t estimates and structural analysis allow an interpretation that eastern Sverdrupfjella, Gjelsvikfjella and CDML comprise a nappe complex, emplaced toward S-SE over western Sverdrupfjella at ca. 500–480 Ma. The timing of the nappe complex emplacement is constrained by 510–490 Ma syn-D3 granitic intrusions and rapid uplift reflected by the 40Ar/39Ar data in Sverdrupfjella. Earlier top-to-the-NW D1-D2 folds and thrust faults preserved in the nappe complex predate the D3 nappe implacement. The emplacement of the nappe complex would position the Urfjell Group source much closer to its basin depository without intervening topography. The reach of the Kuunga Orogeny in DML is limited to central Kirwanveggan, with its southern limit being the Urfjell Group basin at ca. 530 Ma. Planar fabrics defined by hornblende and biotite, with older ages up to ~1100 Ma in southern Kirwanveggan, impliy dominantly Mesoproterozoic deformation in that area. with partial resetting of 40Ar/39Ar ages in northern Kirwanveggan. In contrast, D3 deformation in Sverdrupfjella is Cambrian in age.Integration of geochronological and geochemical data with published aeromagnetic and aerogravity data from WDML support the structural differences observed between Kirwanveggan and Sverdrupfjella and support a depth/heating gradient at ~530 Ma between N. Sverdrupfjella and S. Kirwanveggan.The absence of significant Neoproterozoic deformation in Kirwanveggan and the relatively undeformed nature of the ~530 Ma Urfjell Group rocks and Permian cover rocks, extending from southern Kirwanveggan to Vestfjella, indicate the Kuunga Orogeny is limited to Sverdrupfjella and possibly N. Kirwanveggan but is not continuous to Heimefrontfjella along the margin of the Grunehogna Craton.

40Ar-39Ar dating and petrology of monzonite ejecta in tephra from Quaternary Gölcük volcano (Isparta, SW Turkey): tear-related contrasting metasomatic symptoms in extensional mantle-derived magmas

Eruption of (74-26 ka old) pyroclastic (pumice fall) flow deposits of the Quaternary Gölcük volcano, Isparta, SW Turkey, carried a cargo of Western Anatolian’s youngest monzonitic-syenitic ejecta, 40Ar-39Ar hornblende ages range 313+64−268+43 ka, and indicate that a shallow level felsic plutonic complex existed underneath the crater. These ejecta represent some of the youngest plutonic rocks known. The cooling time of this complex was coeval with the earlier main pyroclastic flow deposits (440 - 148 ka) of the multi-cycle eruptive center which also had a long-lived magma chamber. The monzosyenite ejecta contain plagioclase, K-feldspar, clinopyroxene, amphibole, mica, and accessory apatite, titanite and magnetite. In terms of Nb-Y-3Ga and Nb-Y-Ce, the ejecta, with strong shoshonitic affinity, plot in the field of A1 subtype granites, and represent A1-type magma generation in a back-arc extensional setting. Low Sr and high Nd isotopic ratios of monzosyenites exhibit isotopic similarity to enriched mantle-derived Gölcük-Isparta shoshonitic-ultrapotassic magmas. REE patterns show no significant Eu anomaly, but a strong enrichment of the LREE over the HREE. In multi-element diagrams, however, all samples have negative anomalies of Nb_Ta, Zr_Hf, P and Ti and strong enrichments of Ba, Sr, U, and Th. Moreover, variable (Hf/Sm)N and low (Ta/La)N ratios reflect subduction-related metasomatism in their mantle source. Gölcük monzosyenites and shoshonitic-ultrapotassic rocks overlap with mantle-derived A1-type alkali syenites in trace element abundances. This observation suggests that not only subduction–related fluids/melts, but also intraplate sublithospheric metasomatic agents played a crucial role on their mantle source. It is contended that, due to the percolation of both sublithospheric alkali silicate- and subduction-released-melts/fluids, the metasomatized mantle-derived parental magmas may illustrate a chemical paradox, carrying geochemical imprints of genetically distinct types of mantle sources. Such a metasomatic component in continental lithosphere defined in the genesis of Gölcük monzonitic/syenitic magmas can be explained by tearing of the African oceanic slab during Miocene, where slab-derived and intraplate melts/fluids concurrently modified the lithospheric mantle. Metasomatism of wall-rock peridotites occurred prior to the generation of Gölcük magmas during Miocene slab-tearing. Plio-Quaternary Gölcük potassic magmas were possibly derived from partial melting of carbonated mantle metasomes under an extensional tectonic regime.

40Ar-39Ar age constraints on timing of a late hydrothermal event in the Hutti greenstone belt, Karnataka, India

Obtaining constraints on the timing of orogenic gold mineralization hosted in shear zones in greenstone belts is often difficult because of the lack of easily datable minerals and problems of overprinting of earlier events of mineralization by later hydrothermal events. In the orogenic Hutti gold deposit the U-Pb SHRIMP date obtained from a monazite grain within arsenopyrite that gave an age of 2547 ± 10 Ma, is generally accepted as the age of early hydrothermal mineralization. Nevertheless, other work has indicated that hydrothermal mineralization may have occurred in later episodes as well. This study reports new 40Ar-39Ar ages for hydrothermal biotite, amphibolite and hornblende in granites that occur along the northern and southeastern boundary of the greenstone belt. The 40Ar-39Ar ages of biotite samples are inferred to be the result of re-setting of the ages of alteration biotite by the last phase of hydrothermal activity in this area at ∼2332 Ma. The study indicates that the effects of the thermal event are more significant in areas of greater hydrothermal access such as within the Hutti shear zone. This late hydrothermal alteration event may be related to the D3 deformational event of Roy (1991). The results of the present work are important because it demonstrates a significant hydrothermal event much after the major 2.58–2.52 Ga accretionary event and accompanying TTG and felsic magmatism in the Eastern Dharwar Craton.

Cooling history and age of magnetization of a deep intrusion: A new 1.7 Ga key pole and Svecofennian-post Svecofennian APWP for Baltica

A paleomagnetic and chronogical study has been performed on the Turinge gabbro-diabase formation and on a cross cutting basic dyke in central Sweden and on the Joulovaara gabbro intrusion in northern Sweden in the Fennoscandian Shield. U-Pb age of baddeleyite and 40Ar/39Ar ages of hornblende and biotite reveal a cooling history of the deep gabbro-diabase intrusion in Turinge. The cooling is suggested to have taken place in two stages, one related to the time of intrusion in temperature down to ca 500 °C with a cooling rate up to 46–59°/Ma and another at a lower rate of ca 2.9 °C/Ma, which is suggested to be related with uplift. From this cooling history it can be concluded that the magnetization age of the diabase, ca 1695–1700 Ma, is close to the crystallization age and the 40Ar/39Ar age of hornblende. Applying a similar cooling history for the other studied deep intrusion, the ca 1800 Ma gabbro of Joulovaara gabbro, it is estimated that the magnetization age of the gabbro should be close to that of the U-Pb age of the formation, although the pole of the Joulovaara gabbro is less reliable.The cooling history presented here for the Turinge gabbro-diabase has implications for estimations of magnetization ages also for other deep intrusions.The new pole (Plat. = 51.6°, Plon. = 220.2°; A95 = 4.8°) of the Turinge gabbro-diabase passes most of the reliability criteria and is considered a new key pole for Fennoscandia.The Basic dyke that cuts the Turinge gabbro-diabase was here dated at ca 1200 Ma (whole rock, 40Ar/39Ar) and the virtual geomagnetic pole calculated from its primary magnetization falls into the expected trend of APWP for Baltica.The new Turinge key pole prolong the time of overlapping poles for Fennoscandia, indicating only small movements of the shield between ca. 1870 to 1700 Ma.

Glacial erosion of East Antarctica in the Pliocene: A comparative study of multiple marine sediment provenance tracers

The history of the East Antarctic ice sheet provides important understanding of its potential future behaviour in a warming world. The provenance of glaciomarine sediments can provide insights into this history, if the underlying continent eroded by the ice sheet is made of distinct geological terranes that can be distinguished by the mineralogy, petrology and/or geochemistry of the eroded sediment. We here present a multi-proxy provenance investigation on Pliocene sediments from Integrated Ocean Drilling Program (IODP) Site U1361, located offshore of the Wilkes Subglacial Basin, East Antarctica. We compare Nd and Sr isotopic compositions of <63μm detrital fractions, clay mineralogy of <2μm fractions, 40Ar/39Ar ages of >150μm ice-rafted hornblende grains, and petrography of >2mm ice-rafted clasts and >150μm mineral grains. Pliocene fine-grained marine sediments have Nd and Sr isotopic compositions, clay mineralogy, and clast characteristics that can be explained by mixing of sediments eroded from predominantly proximal crystalline terranes with material derived from inland sources from within the currently glaciated Wilkes Subglacial Basin. Conversely, evidence for such an inland source is absent from ice-rafted hornblende ages. We render a lithological bias against hornblende grains in the doleritic and sedimentary units within the basin the most likely explanation for this observation. 40Ar/39Ar hornblende ages however record additional provenance from the distal margins of the Ross Sea, and possibly even the West Antarctic area of Marie Byrd Land. The latter lies >2000km to the east and hints at significant iceberg release from the West Antarctic ice sheet during warm intervals of the Pliocene. Together our results make a strong case for combining geochemical and mineralogical signatures of coarse- and fine-grained glaciomarine sediment fractions in order to derive robust provenance interpretations in ice covered areas.

LATE MESOZOIC GRANITOIDS OF THE WESTERN TRANSBAIKALIA (RUSSIA) AND THEIR RELATION TO FORMATION OF METAMORPHIC CORE COMPLEXES

Early Cretaceous metamorphic core complexes (MCCs) are widespread in North-East Asia and indicate a large-scale crustal extension in this area [Wang et al., 2011, 2012]. Traditionally one of the formation mechanisms of MCCs is related to various magmatic activities including granitoid magmatism [Anderson et al., 1988, Hill et al., 1995; Lister, Baldwin, 1993]. Wang et al. [2012] have subdivided the intrusion associated with MCCs in NE Asia into pre-kinematic (~170–140 Ma), syn-kinematic (~150–125 Ma) and post-kinematic (~125–110 Ma). 40Ar/39Ar biotite and hornblende ages of 140–110 Ma are overlapping for all MCCs of NE Asia and represent the time of the final stage of the MCCs formation [Wang et al., 2012]. Here, we present overview of geochronological and geochemical data for Late Mesozoic granitoids of the Western Transbaikalia and our view on their role in formation of Transbaikalian MCCs.

Dextral strike-slip along the Kapıdağ shear zone (NW Turkey): evidence for Eocene westward translation of the Anatolian plate

The northern part of the Kapidag Peninsula (Marmara Sea, NW Turkey) is affected by the E–W trending Kapidag shear zone, which cuts through calc-alkaline granitoids of the Ocaklar pluton resulting in mylonitic orthogneiss. Macroscopic and microscopic shear-sense indicators, such as SC fabrics, shear bands, σ-clasts and mica fish, unequivocally suggest dextral strike-slip for the Kapidag shear zone. Based on petrographic data, deformation microfabrics of quartz and feldspar, and the slip systems in quartz, the dextral shearing should have been active at T = 500–300 °C and P < 5 kbar. Published K–Ar and 39Ar–40Ar cooling ages of hornblende and biotite suggest that cooling below 500–300 °C occurred during the Eocene (ca. 45–ca. 35 Ma), meaning that the Kapidag shear zone should have been active during Middle to Late Eocene times. The differential stress related to the shearing was <50 MPa as is indicated by the size of recrystallized quartz grains. Based on the new and published data, it is concluded that the westward movement of the Anatolian plate might have been active almost continuously from the Middle Eocene until recent times.

A 17 Ma onset for the post-collisional K-rich calc-alkaline magmatism in the Maghrebides: Evidence from Bougaroun (northeastern Algeria) and geodynamic implications

Bougaroun is the largest pluton (~200km2) in the 1200km-long Neogene magmatic belt located along the Mediterranean coast of Maghreb. New U–Pb dating on zircons and K–Ar ages on whole rocks and separated minerals document its emplacement at 17Ma within the Lesser Kabylian basement, a continental block that collided with the African margin during the Neogene. This Upper Burdigalian intrusion is therefore the oldest presently identified K-rich calc-alkaline massif in the whole Maghrebides magmatic lineament and marks the onset of its activity. The Bougaroun peraluminous felsic rocks display a very strong crustal imprint. Associated mafic rocks (LREE-enriched gabbros) have preserved the “orogenic” (subduction-related) geochemical signature of their mantle source. Older depleted gabbros cropping out at Cap Bougaroun are devoid of clear subduction-related imprint and yielded Ar–Ar hornblende ages of 27.0±3.0Ma and 23.3±3.2Ma. We suggest that they are related to the Upper Oligocene back-arc rifted margin and Early Miocene oceanic crust formation of the nearby Jijel basin, an extension of the Algerian basin developed during the African (Tethyan) slab rollback. The fact that the Bougaroun pluton intrudes exhumed Kabylian lower crustal units, mantle slices and flysch nappes indicates that the Kabylian margin was already stretched and in a post-collisional setting at 17Ma. We propose a tectono-magmatic model involving an Early Miocene Tethyan slab breakoff combined with delamination of the edges of the African and Kabylian continental lithospheres. At 17Ma, the asthenospheric thermal flux upwelling through the slab tear induced the thermal erosion of the Kabylian lithospheric mantle metasomatized during the previous subduction event and triggered its partial melting. We attribute the strong trace element and isotopic crustal signature of Bougaroun felsic rocks to extensive interactions between ascending mafic melts and the African crust underthrust beneath the Kabylie de Collo basement.

40Ar/39Ar age, lithogeochemistry and petrographic studies of the Cretaceous Alkaline Marapicu Intrusion, Rio de Janeiro, Brazil

The Marapicu Alkaline Massif is an intrusion into the Marapicu-Gericinó-Mendanha Igneous Complex that is part of the Cretaceous Poços de Caldas-Cabo Frio magmatic lineament located in the Southeastern region of Brazil. Nepheline syenites and phonolites are the most abundant rocks in the massif that also include syenites forming an alkaline series SiO2- undersatured. Chemically this series is predominantly metaluminous and to a lesser extent peralkaline. This series presents both potassic and sodic suites being the first one in greater content. The data show that both basic and intermediary rocks with parental composition sampled in this area have no genetic relationship with the other rocks of the body. Geochemistry data shows that evolution processes involved fractional crystallization with or without continental crust assimilation and also indicates that this alkaline magma was generating from an enriched mantle source. The 40Ar/39Ar age of hornblende (extracted of nepheline syenite) from Marapicu massif is 80.46 ± 0.58 Ma, which it is contrasting, with the idea of age decrease of the hotspot track from west to east on the Poços de Caldas-Cabo Frio magmatic lineament.

Tracking the provenance of Greenland-sourced, Holocene aged, individual sand-sized ice-rafted debris using the Pb-isotope compositions of feldspars and 40Ar/39Ar ages of hornblendes

The provenance of sand-sized ice-rafted debris (IRD) sourced from Greenland is currently difficult to determine. Such knowledge, if it could be ascertained with a high degree of certainty, could be applied to the Greenland-proximal marine records to improve both our understanding of modern-day spatial patterns of iceberg rafting and the past history of the Greenland Ice Sheet (GIS). Recent studies have highlighted the utility of the Pb-isotope composition of individual sand-sized feldspars and the 40Ar/39Ar ages of individual sand-sized hornblendes in this regard. However, before any such provenance toolkit can be applied to the palaeo-record, it is necessary first to determine whether this approach can be used to track the sources of known recent Greenland-proximal IRD deposition. To this end we present new records of the Pb-isotope composition and the 40Ar/39Ar ages of individual sand-sized grains of feldspars and hornblendes, respectively, from modern Greenland glacifluvial and fjord sands and Holocene to modern Greenland-proximal marine sediments. These new data demonstrate that sand-sized feldspars and hornblendes glacially eroded by the GIS exhibit distinct intra- and inter-tectonic terrane differences in their Pb-isotope compositions and ages and that these differences are clearly expressed in the geochemistry and geochronology of sand-sized IRD deposited in marine sediments around Greenland. Although overlap exists between some Greenland-proximal IRD ‘source fields’ defined by these data, our approach has the potential to both better understand spatial patterns of Greenland-derived IRD in the modern day as well as during past episodes of iceberg calving.

Kinematics of the Tengchong Terrane in SE Tibet from the late Eocene to early Miocene: Insights from coeval mid-crustal detachments and strike-slip shear zones

It is generally believed that the extrusion of SE Tibet was bounded by the dextral Gaoligong and the sinistral Ailaoshan-Red River strike-slip shear zones from the Oligocene to early Miocene. This study integrates field mapping, structural analysis and geochronology in western Yunnan (China), where foliated Precambrian basement rocks and late Cretaceous to early Eocene plutons are exposed to the west of the Gaoligong shear zone. We found that late Eocene to early Miocene flat-lying ductile shear zones were kinematically related to steeply dipping strike-slip shear zones. Four elongated gneiss domes (Donghe, Guyong, Yingjiang and Sudian) are cored by high-grade metamorphic rocks and pre-kinematic granite plutons, and bounded by top-to-NE detachments and NE-trending dextral strike-slip shear zones. Zircon U-Pb ages from LA-ICP-MS analysis and 40Ar/39Ar ages of micas and hornblende demonstrate that the flat-lying Donghe Detachment (>35–15Ma) and the Nabang dextral strike-slip shear zone (41–19Ma) were sites of prolonged, mostly coeval ductile deformation from amphibolite to greenschist facies metamorphism. The Gaoligong shear zone experienced dextral shearing under similar metamorphic conditions between 32 and 10Ma. Consistent 40Ar/39Ar ages of hornblende from the three shear zones indicate their contemporaneity at mid-crustal depth, causing the rapid exhumation and SW-ward extrusion of the Tengchong Terrane. The strain geometry and shear zone kinematics in the Tengchong Terrane are interpreted with folding of the anisotropic lithosphere around a vertical axis, i.e., the northeast corner of the Indian Plate since 41Ma. The newly discovered NE-trending Sudian, Yingjiang, and Lianghe strike-slip shear zones are subordinate ductile faults accommodating the initially rapid clockwise rotation of the Tengchong Terrane. The detachments caused mid-crustal decoupling and faster SW-ward extrusion below the sedimentary cover, whereas the strike-slip shear zones accommodated extrusion and clockwise rotation of the Tengchong Terrane around the proto-Eastern Himalayan syntaxis since the late Eocene.