Orogenic Massifs Research Articles

Fractionation of Platinum-group Elements and Gold in the Upper Mantle: a Detailed Study in Pyrenean Orogenic Lherzolites

Platinum-group elements (PGE) and gold have been analysed in INTRODUCTION 68 mantle peridotite samples (lherzolites, cpx-poor lherzolites and Platinum-group elements (PGE) are classified, along with harzburgites) from Pyrenean orogenic massifs. These samples preserve Au and Re, as highly siderophile elements. Pioneering PGE and Au distribution patterns mostly inherited from a 2-Gastudies on basalt-hosted mantle xenoliths (see Jagoutz et old low-degree, partial melting event. The light rare earth element al., 1979; Mitchell & Keays, 1981; Morgan et al., 1981) (LREE)-depleted lherzolites display a remarkably homogeneous suggested that these ‘iron-loving elements’ are overPGE and Au concentration range (0·005 to 0·012× CI chonabundant in the Earth’s mantle compared with theoretical drites). The light PGE (Pd, Rh, Ru) are slightly enriched relative values predicted by core–mantle equilibrium partitioning to the heavy PGE and CI chondrites [(Pd/Ir)N = 1·8 ± 0·2; models (e.g. Borisov et al., 1994). Moreover, highly si(Pd/Pt)N = 0·8 ± 0·2]. The harzburgites are Au and Pd derophile elements occur in approximately chondritic depleted [(Pd/Ir)N < 1; (Au/Ir)N < 0·2] in addition to being Cu relative abundances. These two features have been used and S depleted relative to fertile lherzolites. These coupled variations to support the ‘late-veneer hypothesis’ according to which indicate that Pd and Au were removed along with a sulphide melt. highly siderophile elements were reintroduced into the The contents of Ir, Ru, Rh and sometimes Pt are constant over the mantle by the intense meteoritic bombardment that hit whole range of peridotite compositions (0·6 < Al2O3 < 4·2 wt %), thus reflecting the compatible behaviour of these elements in lowthe primitive Earth after core formation (Chou, 1978; degree mantle melting processes. Ruthenium and rhodium show Morgan, 1986; O’Neill, 1991). Although consistent with similar concentrations to Pd in lherzolite-hosted Cu–Fe–Ni sulphides the chondritic Os isotopic composition inferred for the but their whole-rock abundances do not decrease from lherzolites to primitive upper mantle (Meisel et al., 1996), the late sulphide-free harzburgites. To a lesser extent, this is also true for veneer hypothesis has been contradicted by recently Pt and Ir. Such behaviour suggests that, except for Pd and Au, the published PGE data. Pattou et al. (1996) identified higher PGE reside in mantle sulphides as melting-resistant atomic clusters than chondritic Pd/Ir and Rh/Ir ratios in fertile Pyrenean or micro alloys. Pd, Rh and Ru enrichment relative to Ir similar orogenic lherzolites. Likewise, Snow & Schmidt (1998) to that observed in Pyrenean orogenic lherzolites has now been found Ru/Ir, Rh/Ir, Pt/Ir and Pd/Ir ratios about 40% identified in all the orogenic lherzolites analysed so far for PGE, higher than those of the CI chondrites in abyssal periin abyssal peridotites and a few basalt-hosted mantle xenoliths. dotites. This is inferred to be a characteristic feature of the shallow midHigher than chondritic Ru/Ir, Rh/Ir and Pd/Ir ratios ocean ridge basalt source mantle since at least the early Proterozoic. have now been identified in all the orogenic peridotite

MANTLE REDOX AND CONTRASTING TECTONIC PROVENANCE FOR OCEANIC AND CRATONIC MANTLE LITHOSPHERE

The compositional and mineralogical distinction between mantle lithosphere underlying old continental cratons and that beneath orogenic belts and ocean basins is well established (Boyd, 1989). The origin of many orogenic and oceanic peridotites as residues of partial melting at pressures less than 3 GPa in the spinel stability field is generally accepted. At such pressures, residues will be Ol-rich. In contrast, the highly depleted character, yet relatively Olpoor (and Opx-rich) mineralogy recognized in xenoliths from many cratons require that such lithosphere is not a simple residue. Hypotheses for the formation of cratonic lithosphere were reviewed by Kelemen et al. (1998) who provided compelling evidence that most of the mantle lithosphere beneath cratons originates by partial melting in the spinel stability field, and that cratonic garnet peridotites with higher equilibration pressures have been tectonically transported to such depths. In this way, ‘on-craton’ lithosphere could be considered oceanic peridotite that has underplated to form Archean continents with the deep lithospheric keels. Nonetheless, of the hypotheses reviewed by Kelemen et al. (1998), none have convincingly tied the petrological origin of cratonic mantle lithosphere to a specific tectonic setting. It has been speculated that Opx-enrichment in cratonic mantle resulting from melt/rock reaction would proceed in a subduction zone, whereas the same feature would occur from high degrees of melting and related cumulate enrichment in the root of a mantle plume. Distinct chemical or mineralogical lines of evidence to link the mantle samples to these tectonic environments is so far lacking. When interpreted in the light of a growing experimental data base for V partitioning (Canil, 1997; 1999) the systematics for V in cratonic and oceanic mantle help to further distinguish the conditions and the tectonic setting during melt depletion to form these two contrasting types of lithosphere. Examination of a data base of 200 spinel- and garnet peridotite analyses of on- and off-craton mantle xenoliths, and orogenic massifs from various settings show that the V abundances in mantle lithosphere lies along two distinct coherent trends (see Figure) when plotted against a depletion index such as wt% MgO. For a given degree of depletion, the V abundances of all cratonic peridotites, whether spinel- or garnet-bearing, are at lower levels than those of oceanic peridotites. The covariation of V with depletion for oceanic peridotites is explained (and modelled) as partial melting in the spinel stability field. Spinel has a DV that is ~ 10 times higher than any other peridotite mineral at a given oxygen fugacity (fO2), and as a residual phase during partial melting at low/moderate pressures, and fO2’s near that of MORB generation (NNO- 2 or -3), will dominate the budget for V. On the other hand, the spectrum of cratonic mantle compositions in the Figure requires either of the following: (1) Oceanic and cratonic peridotites represent residues from two completely different primitive mantle (PUM) sources; the source for cratonic mantle having a lower overall V abundance relative to PUM. (2) Unlike oceanic peridotites, cratonic peridotites are residues of melting at high pressures in the garnet stability field. Because DV Gt is less than DV Sp, less V is retained in the residue for a given degree of depletion. Quite possibly, neither garnet nor spinel are residual in the original melt depletion events related to cratonic peridotites, and both pyroxene and/or olivine, having low DV are responsible for the smaller amount of V in the residues. (3) Cratonic mantle originates by depletion in the Sp stability field at low pressure where Sp is a residual phase, but at much higher fO2 than oceanic peridotites related to MORB generation. The DV for Sp is very fO2 sensitive and varies almost an order of magnitude over the range of terrestrial fO2’s. The lower DV Sp during melting at higher fO2 (near NNO) would result in less V being retained in the residue. Hypothesis (3) would constrain the formation of cratonic peridotites to a region of melt generation in the mantle of relatively high FO2, most likely related to subduction. The distinctly lower Ti/V ratios recognized in many arc-related tholeiites (Shervais, 1982) are a complimentary signature related to this process. The above hypotheses will be quantitatively described and explored using appropriate peridotite melting models and partition coefficients. If hypothesis (3) is accepted then partial melting in the Archean was more oxidized than hitherto realized, some form of subduction was operative, and the mantle lithosphere beneath Archean cratons is intimately related to the subduction process.

THE GENESIS OF PYROXENITE-RICH PERIDOTITE AT CABO ORTEGAL (NW SPAIN). INFERENCES FROM GEOCHEMICAL, MINERAL AND PB-SR ISOTOPIC DATA

THE GENESIS OF PYROXENITE-RICH PERIDOTITE AT CABO ORTEGAL (NW SPAIN). INFERENCES FROM GEOCHEMICAL, MINERAL AND PB-SR ISOTOPIC DATA

SULFUR ENRICHMENT VIA VOLATILE-RICH METASOMATISM IN MONTFERRIER XENOLITHS (SOUTHERN FRANCE): CONSEQUENCES FOR HIGHLY SIDEROPHILE ELEMENTS IN THE SUB-CONTINENTAL MANTLE

Sulfides are the main hosts for highly siderophile elements (HSE); therefore the mantle S content is a crucial datum for geochemical studies relevant to platinum-group element abundances or Re-Os systematics. The estimated S content of the primitive mantle is about 250±50 ppm. Although S contents of fertile peridotite from orogenic massifs support this estimate, peridotites hosted in alkali basalt commonly contain only a few tens of ppm S, whatever their fertility (Lorand, 1990). Some xenoliths from southern France, i.e. Montferrier volcano (Herault) have very high sulfide abundances (up to 0.2 wt.%) with intergranular sulfides predominating over enclosed sulfides (Lorand and Conquere, 1983). The process responsible for such high sulfide modal abundances and its possible effect on HSE abundances are not clear. To shed some light on this process we have carried out an extensive study of 21 xenoliths from this locality, involving major elements and lithophile trace element data, chalcogenide (S, Se) geochemistry, platinum-group element analyses and Re-Os systematics. Montferrier xenoliths are either breccia-hosted or lavahosted. The former have a yellowish color indicating weathering, while the lava-hosted have a fresh green color and do not show obvious alteration features. Some xenoliths display disseminated Ti-pargasite (up to 6%) regardless of their host. Montferrier peridotites are characterized by a strongly deformed porphyroclastic microstructure giving them a mylonitic appearance. It has been shown that this deformation took place at low temperature (900-750°C), probably during an uplift on the nearby Cevennes lithospheric shear zone (e.g., Fabries et al., 1987). They also display very fertile compositions. Most of the studied samples contain between 3.0 and 4.5% Al2O3 and the least fertile still contains 2.0 wt.% Al2O3. The S concentration range (7 - 592 ppm) is the highest reported for basalt-hosted peridotite xenoliths. S contents are uncorrelated with fertility indices (e.g., Al2O3%). Breccia- hosted xenoliths have the lowest S contents ( 10). Ba and Rb enrichments are determined by amphibole. The second end-member is strongly enriched in LREE (La/SmN > 1). This LREE enrichment, is accompanied by large ion lithophile elements (Sr, U, Th...), but without concomitant enrichment of the high field strength elements (Nb, Ta, Zr, Hf). In contrast to the depleted type, the U/Th ratios are chondritic. Such enriched patterns are commonly ascribed to mantle metasomatism by carbonated and/or volatile-rich small volume melts. The positive correlation of figure 1B indicates that large amounts of S have been added by this metasomatism. In thin sections, the xenoliths containing more than c.a. 200 ppm S show numerous large intergranular sulfide blebs (up to 700 µm in maximum dimension) much richer in Fe and Cu than those from the depleted samples. Contrary to S, Se contents increase only moderately from 17-48 ppb in depleted samples to 50- 63 ppb in enriched xenoliths. The most S-rich samples may show markedly higher than chondritic S/Se ratios (104) which are inconsistent with a simple addition of immiscible sulphides during metasomatism. This observation suggests that S was a dissolved species in the volatile-rich metasomatic fluid whose effects are indicated by the trace element studies. Apart from the most refractory lherzolite which is unusually poor in PGE (total PGE = 5.2 ppb), the 8 xenoliths analyzed for the PGEs show a narrow PGE concentration range (20-28.2 ppb) independant of their fertility indices and S contents. Ru/Ir and Rh/Ir ratios are remarkably constant and slightly higher than chondritic. Pd/Ir ratios (1.37-2.26) are suprachondritic, as reported for Pyrenean orogenic lherzolites (Pattou et al., 1996) and some abyssal peridotites (Snow and Schmidt, 1998, Luguet et al., this volume). Pd/Ir ratios are correlated with S contents, mostly because the enriched samples have low Ir and Ru contents. Whole-rock fractionated Pd/Ir ratios are reflected in the pattern of the sulphides (LA-ICP-MS data). Os/Ir varies sympathetically with Pd/Ir, increasing from nearly chondritic values (1.1) in the depleted sample to suprachondritic (1.99) in the most S-rich xenolith. 187Os/188Os shows a similar increase from DMM-like values (0.128) in the depleted samples to highly radiogenic values (0.175) in the most S-rich sample. Our study provides further evidence that metasomatic process(es) involving volatile-rich small melt fractions may significantly alter the HSE relative abundances and Os isotopic composition of the lithospheric mantle (cf Gutierez Narbonna et al., this volume). Such parallel variations of Os, Pd and S are consistent with the experimentally-demonstrated affinity of these elements for S-bearing vapor phases at magmatic temperatures.

Re-Os Systematics in Orogenic Peridotite Massifs and Constraints on the Petrogenesis of Pyroxenites

Orogenic lherzolites are considered to represent large portions of the upper mantle that have been tectonically emplaced in the crust. They are composed of lherzolites, harzburgites, dunites and small proportions of pyroxenites of variable thickness and composition. The study of such massifs has provided much information on the spatial distribution of geochemical variations in the upper mantle. Because of the extreme compatibility of osmium and its resistance to secondary effects like metasomatism and retrograde alteration, The Re-Os isotopic system provides an additional perspective to help understanding the origin of orogenic lherzolites massifs. Here we investigate, Re-Os concentrations and isotopic compositions of peridotites and pyroxenites from three ultramafic massif: Beni-Bouzra (North Morocco), Ronda (South Spain) and Lherz (French Pyrenees). The Os isotopic ratios (1870S/1860S) for the peridotites range from 0.951 to 1.102 in good agreement with data published previously and with upper mantle value, while the concentration range from 0.531 to 8.740 ppb and is in general higher than those encountered in mantle xenoliths. It is widely admitted that orogenic massif come from the asthenospheric mantle that accreted into the lithosphere sub-continental (SCLM) via limited diapiric upwelling. Subsequently the correlation between Os isotopic ratios and Mg# or A1203 may result from ancient melt extraction (Fig. 3). Our study of mafic layers has focused on two familly of pyroxenites: spinel websterites and garnet pyroxenites. The garnet pyroxenites are more differentiated than spinel websterites and have high concentration in A1203, CaO end V and low concentration in MgO, Ni and Co. The osmium isotopic ratios of garnet pyroxenites range from 1.752 to 12.352 (Fig. 2). These values are higher than those encountered in the websterites (from 1.058 to 1.904) and are the result of a large Re/Os fractionation (160 to 900). Among the mafic samples, Os isotopic ratios are found to decrease strongly with increasing Os concentration. The negative correlation obtained between Re/Os and Mg# confirms the incompatible nature of Re. The pyroxenites data of Beni-Bouzra show a positive correlation (R 2 = 0.97, Fig. 1) between 1870s/1860s and lS7Re/lS6Os ratios which yield an apparent 'age' of 700 _+ 55 Ma. This age is in agreement with the model age obtained with 2~176 systematics for the same massif (590 +_ 150 Ma; Hamelin and Allbgre, 1988) and may reflect the time when was incorporated in the non-convecting SCLM. This event is younger than the stabilisation of, the Ronda ultramafic massif deduced for Os isotopes(~ 1.2 Ga, Reisberg and Lorand, 1995). The high Os isotopic ratios in mafic layers suggesting that this material cannot represent recent partial melting, or cumulates derived from recent melt of the surounding ultramafic rocks. They provide evidence for a process that occurred at least hundreds of millions of years ago. Comparison of the Os and Pb data for the peridotites and pyroxenites with the range observed in oceanic basalts shows that the radiogenic ratios in basalts could be produced by a mixture ofperidotitepyroxenite in the source which lends support to the hypothesis that the lithologies found in orogenic massifs are representative of the upper mantle, the 'marble cake' model (All~gre and Turcotte, 1986). However, the geochemical characteristics of pyroxenites, the zonation observed in some mafic layers and the positive correlation observed between the model age versus thickness (Fig. 4) suggests that this material represent high-pressure crystallization from melts of subducted oceanic crust (Pearson et al., 1993). Although in the basis of FeO-MgO-AI203 systematics, small proportion of Beni-Bouzra pyroxenites may represent subducted, metamorphosed oceanic crust directly recycled into the mantle (Kornprobst et al., 1991).

Further Insights on S Content and Behaviour in the Lithospheric Mantle

Sulphides are minor but common phases in mantle rocks. They are thought to be the main mineral host in mantle rocks for highly siderophile elements (HSE). Therefore, sulphide mineralogy and S concentration are key factors for understanding the behaviour of PGEs and the Re/Os isotopic system in the mantle. Previous studies have shown extremelly different S contents for the two main types of upper mantle samples, i.e. orogenic massifs and xenoliths. The massifs have S contents in the same order of magnitude as estimated for MORB source (~200 ppm, e.g. O'Neil, 1991) and correlated with major element contents. In contrast, peridotite xenoliths brought up by alkali basalts generally have low S contents (~ cpx%~>19) , bu t a few are o l r i c h (75>O1%~>97). Mt Gambier peridotites are more depleted, including harzbugite and cpx-poor lherzolites (0>cpx%>9); two are cpx-rich with 14 and 25% of cpx. Xenoliths from Wallabadah Rocks range from harzbugites to lherzolites (4>cpx%>15) and Allyn River samples are mainly lherzolitic (cpx%~16). Montferr ier xenol i ths are fert i le lherzol i te (1 l>cpx%>15) displaying highly deformed porphyroclastic microtexture and rare disseminated pargasite. Among the Montferrier xenoliths two populations can be distinguished one consist of fresh dark-green xenoliths in basaltic lava flow, the second one of altered yellowish xenoliths within a breccias. Sulphur contents were measured using iodometric method at the Museum National d'Histoire Naturelle, Paris, Cu and trace element analysis were performed on ICP-MS at Macquarie Universi ty or at Montpellier II University. Mt Gambier xenoliths have higher sulphide modal abundances than Mt Quincan samples. Sulphides in Mt Gambier xenoliths occur as Isolated blebs fully enclosed in olivine, spheroidal bodies up to 400 gm in diameter at spinel-silicate junctions, small droplets along fluid inclusion trails, in vermicular intergrowths at the silicate grain boundaries or in sulphide veins penetrating the silicates. Weathering is not uncommon but of very limited extent (0-30%). In contrast, Mt Quincan xenoliths contain only a few sulphide blebs (50-300 gin), generally highly weathered (30 to 90%) per polished thin section. Wallabadah peridotites have very variable sulphide habits and abundances, and the degree of weathering is variable (up to 50%). Allyn River peridotites show very unusual numerous large interstitial sulphides with angular shapes and 'jagged' rims. Montferrier xenoliths have large sulphides (up to 300 gm) with polygonal shapes in microtextural equilibrium with the neoblastic matrix. These sulphides are highly altered (50 to 80%) in breccia-hosted periotite and almost unaltered (<15%) in lava hosted-xenoliths. The common sulphide assemblage in all the xenolith suites is pentlandite, pyrrhotite, monosulphide solid solution (mss) and chalcopyrite.

Distribution of Trace Elements in Minerals from Anhydrous Spinel Peridotites and Websterites From the Ronda Peridotite: Implications for the Nature of LILE, REE and HFSE Reservoirs in the Subcontinental Lithospheric Mantle

Intermineral distribution coefficients (DXt/r We report dissolution ICP-MS analyses ofRb, Ba, Th, U, Nb, Ta, REE, Sr, Zr, Hf and Sc in leached separates of clinopyroxene, orthopyroxene, olivine and spinel in anhydrous spinel peridotites (12 samples) and websterites (6 samples) from the Ronda peridotite (S. Spain). None of the selected samples display evidence of cryptic metasomatism. This data-set provides a good example of the distribution of a large group of trace elements in anhydrous minerals from the subcontinental lithospheric mantle, as exemplified by orogenic massifs. The average partition coefficients (relative to clinopyroxene: D xt/cpx) of the studied trace elements are shown in Fig. 1. For most elements, there are no statistical differences in the D ~ between spinel peridotites and websterites. For the REE, Zr and Hf, the small standard deviation is suggestive of the lack of an important compositional effect in the partitioning of the studied compositions ('fertiles' to 'depleted'). As previously suggested, clinopyroxene is the main host of REE and other incompatible elements in anhydrous spinel mantle rocks. The D ~ for Zr and Hf is a factor 10 higher than for MREE, indicating the potential of orthopyroxene to fractionate these High Field Strength Elements (HFSEZr, Hf, Nb, Ta) relative to REE 1. When plotted in increasing order of compatibility in a melt/peridotite system (Fig. 1), the D xt/cpx of the Highly Incompatible trace Elements (HIE = Rb, Ba, Th, U, Nb, Ta) systematically increases from the MREE (Fig. 1). This feature is more marked in the olivine/cpx and spinel/cpx partitioning (Fig.l). Although the distribution of MREE, HREE, Zr and Hf in clinopyroxenes, orthopyroxenes and, partly, in olivines might be accounted for by crystallographic control in these minerals. However, the distribution of the HIE is inconsistent with a crystallographic control, especially in the case of spinels and olivines. The fact that the relative increase of the D xtjcpx of the HIE is correlated with their degree of incompatibility in a melt/peridotite system, and that olivines and spinels from different samples display similar normalized trace element patterns and abundances of HIE, indicates that an important part of their budget may be hosted in melt/fluid inclusions within these minerals. In Ronda minerals, two-phase fluid/(melt?) inclusions (~ lgm) typically occur as inclusions trails similar to secondary fluid/melt inclusions documented in peridotite xenoliths worldwide 2.

Platinum-group elements as petrological indicators in mafic-ultramafic complexes of the central and southern Urals: preliminary results

Platinum-group elements (PGE) and gold were analyzed by ICP-MS after the Ni-sulphide fire assay pre-concentration step in 44 rocks from 5 orogenic mafic-ultramafic massifs of the central and southern Urals. They comprise: (a) ring-zoned, Ural-Alaskan type bodies of the ‘Pt-bearing belt’ (Kachkanar, Tagil, Uktus); and (b) lherzolite-harzburgite complexes overlain by layered wehrlite, gabbro, and diorite (Nurali, Mindyak) supposed to be ophiolite-type suites. Total PGE and Au contents are generally higher in Ural-Alaskan type complexes compared with lherzolite-harzburgite complexes. Mantle-normalized PGE patterns range from nearly unfractionated in the dunites ( Pd Ir = 0.57–3.68 ) to mildly and strongly fractionated in wehrlite, clinopyroxenite, and gabbro ( Pd Ir = 9.1–187 ). Exceptionally high values of Pd Ir (761–2622) are observed in amphibole-rich lithologies at Kachkanar, possibly indicating a relationship between fractionation of the less refractory Pd and increased fluid activity in the parental magma. Pronounced, positive Pt-anomalies, up to 20 times the mantle estimate, distinguishes the Ural-Alaskan type dunites, and progressively reduces in pyroxenites and gabbros. In the lherzolite-harzburgite complexes of Mindyak and Nurali, layered wehrlites and pyroxenites, as well as rocks from the gabbro-diorite units have fractionated PGE patterns ( Pd Ir = 2.94–92.39 ) and positive Pt-anomaly as in the Ural-Alaskan complexes. Conversely, the Pt-anomaly is negative in the peridotites, and increases from lherzolites and harzburgites, to dunites, similarly to residual mantle from alpine-type orogenic massifs. The Pd Ir (0.71–3.7) is higher than in ophiolites, and resembles that of lherzolite massifs of the western Mediterranean, representing sub-continental mantle. Variation of the Pt-anomaly versus Pd Ir in Ural-Alaskan type complexes and in the layered sequences overlaying lherzolite-harzburgite mantle suites is consistent with magmatic fractionation, in which Pt behaves as a compatible element. It reflects a progressive depletion by partial melting in mantle peridotites, the Pt becoming incompatible in the last stage of melting in which Pt-poor residual dunites are formed. The positive Pt-anomaly in Ural-Alaskan type complexes is striking, and would appear as a distinctive feature of the parent magma, probably inherited from its mantle source.

Structural and seismic evidence for intracontinental subduction in the Peter the First Range, Central Asia

Research Article| April 01, 1992 Structural and seismic evidence for intracontinental subduction in the Peter the First Range, Central Asia MICHAEL W. HAMBURGER; MICHAEL W. HAMBURGER 1Department of Geological Sciences, Indiana University, Bloomington, Indiana 47405 Search for other works by this author on: GSW Google Scholar DANIEL R. SAREWITZ; DANIEL R. SAREWITZ 2Committee on Science, Space, and Technology, 2320 Rayburn Office Building, Washington, D. C. 20515 Search for other works by this author on: GSW Google Scholar TERRY L. PAVLIS; TERRY L. PAVLIS 3Department of Geology and Geophysics, University of New Orleans, New Orleans, Louisiana 70118 Search for other works by this author on: GSW Google Scholar GYORGI A. POPANDOPULO GYORGI A. POPANDOPULO 4Institute of Physics of the Earth, Academy of Sciences of the USSR, B. Gruzinskaya 10, Moscow 123810 USSR Search for other works by this author on: GSW Google Scholar Author and Article Information MICHAEL W. HAMBURGER 1Department of Geological Sciences, Indiana University, Bloomington, Indiana 47405 DANIEL R. SAREWITZ 2Committee on Science, Space, and Technology, 2320 Rayburn Office Building, Washington, D. C. 20515 TERRY L. PAVLIS 3Department of Geology and Geophysics, University of New Orleans, New Orleans, Louisiana 70118 GYORGI A. POPANDOPULO 4Institute of Physics of the Earth, Academy of Sciences of the USSR, B. Gruzinskaya 10, Moscow 123810 USSR Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1992) 104 (4): 397–408. https://doi.org/10.1130/0016-7606(1992)104<0397:SASEFI>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation MICHAEL W. HAMBURGER, DANIEL R. SAREWITZ, TERRY L. PAVLIS, GYORGI A. POPANDOPULO; Structural and seismic evidence for intracontinental subduction in the Peter the First Range, Central Asia. GSA Bulletin 1992;; 104 (4): 397–408. doi: https://doi.org/10.1130/0016-7606(1992)104<0397:SASEFI>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract This paper examines the stratigraphy, structure, and seismicity of the Peter the First Range, an actively deforming foreland basin in Soviet Tadjikistan. The range represents a highly deformed segment of a large intracontinental basin, the Tadjik Depression, which is being shortened in response to the Cenozoic convergence between two orogenic massifs, the Pamir and the Tien Shan Ranges. The Late Jurassic to Paleogene stratigraphy of the Peter the First Range includes more than 2,000 m of shallow marine to nonmarine sediments deposited under relatively quiescent conditions, close to sea level. In contrast, the correlative units of the northernmost Pamir, directly south of the Peter the First Range, show evidence of proximity to a major, uplifted source area to the south. These Mesozoic-early Cenozoic strata are overlain by thick sequences of alluvial sandstone and conglomeratic units that coarsen upward through the Neogene and record the progressive uplift of the Pamir. Shortening within the Peter the First Range is recorded by kilometer-scale isoclinal, upright-to-over-turned folds, and imbricate, north-verging thrusts that emerge near the northern edge of the range and mark the active tectonic boundary with the Tien Shan. Most of the deformation appears to postdate the accumulation of the Mio-Pliocene molasse. The stratigraphy and structure of the region are best explained by a model of the Mesozoic Tadjik Depression as an intracontinental, foreland basin, developed in a back-arc position with respect to the active Andean or collisional orogens located in northern Afghanistan and the Pamir. Seismicity in the region is dominated by earthquakes extending from the surface to ∼12-km depth, apparently occurring within the deformed sediments of the Peter the First Range. A subhorizontal zone of relatively low activity at this depth may mark the location of a basal detachment that underlies a structurally thickened sedimentary section. This aseismic zone is in turn underlain by a south-dipping belt of seismicity that extends from about 17- to 35-km depth, and which can be traced updip to the seismogenic crystalline basement of the Tien Shan range to the north. We interpret this south-dipping structure as a zone of intracontinental subduction that may be an updip continuation of the enigmatic, intermediate-depth Wadati-Benioff Zone beneath the Pamir Range to the south. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Orogenic Massifs: Protolith, Process and Provenance

Orogenic or alpine massifs comprise clinopyroxene-rich (lherzolite) and clinopyroxene-poor (harzburgite) peridotites with associated concordant (old) and discordant (young) mafic layers. The protolith is believed to be residual peridotite produced by partial melting processes in post-Archaean times and the mafic layers are interpreted as polybaric fractionates of tholeiitic and alkaline melts (amphibole yroxenites/pyroxenites) or recycled products (aluminous pyroxenites). Silicate melt migration in the wall rock peridotite around mafic layers has resulted in the formation of metasomatic and enrichment fronts. In most massifs the peridotite protolith is interpreted as stabilized sub-continental lithosphere, aluminous pyroxenites as recycled oceanic (?) lithosphere, and other mafic layers as evidence of melt ingress from the asthenosphere. Provenance solutions include thinned post- Archaean continental lithosphere, similar to the Basin and Range, or a continental-oceanic transition as exists at present in the Red Sea.

Development of block structure in the crust below the Pannonian basin

The analysis of geophysical and geological data on the structure of the pre-Miocene substratum of the Pannonian basin has revealed a block structure. The crust was divided by deep-seated faults of mainly NE-SW orientation. The deep-seated faults separate zones of continental crust (with granitoids), which were more intensely consolidated during the Hercynian, from strips with volcanic-sedimentary rocks of eugeosynclinal character (sub-oceanic crust). In geological development of the Pannonian basin crust during the last 330 million years, subsidence predominated over uplift and denudation. The great mobility of the crust ist related to its lesser thickness compared to the thick crust in the orogenic border and median massifs, e. g. of the Balkan region. The block structure is superimposed on the original belt structure and nucleus stage of continental crust formation. In the pre-Hercynian stage, tectonic division along E-W and N-S lines predominated. The long persistence and magmatic activity of deep-seated faults and their almost vertical dip do not support proposed models of partial subduction and closing of microoceans in the region of the Pannonian basin. Strips or blocks of suboceanic crust are considered to be relicts of more weakly sialized original thin oceanic crust. The Pannonian basin is not a typical ensialic basin. Tectonic development has distinctly changed the mantle diapir, which originated in the Late Cretaceous. Along the circumference of this diapir, shear zones originated in the crust, along which seismic activity has persisted to the present. The Pannonian megablock is a type of simatic median massif, preserving long-lasting subsidence mobility. The Pannonian mantle diapir is considered to be an autonomous deep structure in the sense ofvan Bemmelen (1972). We do not relate the ascent of the mantle diapir to subduction of lithospheric plates, which must have been the cause of folding of the Carpathians according toStegena et al. (1975).