Muharrem Satir

Tectonics of an ultrahigh-pressure metamorphic terrane: The Dabie Shan/Tongbai Shan Orogen, China

Ultrahigh-pressure metamorphic rocks with coesite and diamond form a tectonic slice over 20 km thick, called the eclogite zone, within the Dabie Shan complex in the Qinling orogen in central China. The orogen separates the Sino-Korean block in the north from the Yangtze block in the south. The Dabie Shan Complex is a composite terrane made up of eclogite facies and amphibolite facies gneiss slices and represents fragments of the lower continental crust of the Yangtze block. The Dabie Shan Complex is bounded in the south by a Triassic foreland fold-thrust belt and in the north by a greenschist facies metaclastic unit, the Foziling Group, which probably represents the passive continental apron deposits of the Yangtze block. Farther north is a granulite facies gneiss complex, the Qinling Group, which has ultramafic slivers and includes the remnants of an island arc with two bounding suture zones. North of the Qinling Group are early Paleozoic active margin deposits of the Sino-Korean block. The eclogite zone in the Dabie Shan Complex is sandwiched between amphibolite facies gneiss slices. Dating by Sm-Nd, Rb-Sr, and Ar-Ar of two eclogite samples from the eclogite zone gives early to middle Triassic ages (236–246 Ma); the initial eNd values indicate reworking of a 2.11 and 1.55 Ga continental crust. A Himalayan-type tectonic evolution is envisaged for the Qinling orogen with the creation of a 100-km-thick crustal thrust wedge through continuous underplating during the subduction of the Yangtze continental lithosphere. Exhumation of the ultrahigh-pressure metamorphic rocks was chiefly achieved by the southward propagation of the thrust planes, thereby isostatically uplifting and eroding the earlier deeply subducted parts of the orogen. A total of 680 km of southward thrusting in front of Dabie Shan is inferred, based on the abrupt termination of the Tanlu fault. Normal faulting possibly caused by gravitational collapse probably also had a role in the exhumation process.

Coeval plutonism and metamorphism in a latest Oligocene metamorphic core complex in northwest Turkey

A metamorphic core complex of latest Oligocene age crops out in the Kazdag˘ mountain range in northwest Turkey. The footwall of the core complex consists of gneiss, amphibolite and marble metamorphosed at 5 ± 1 kbar and 640° ± 50 °C. The average muscovite and biotite Rb/Sr ages from the gneisses are 19 Ma and 22 Ma, respectively, and imply high temperature metamorphism during latest Oligocene times. The hangingwall is made up of an unmetamorphosed Lower Tertiary oceanic accretionary melange with Upper Cretaceous eclogite lenses. The hangingwall and footwall are separated by an extensional ductile shear zone, two kilometres thick. Mylonites and underlying high-grade metamorphic rocks show a N-trending mineral lineation with the structural fabrics indicat- ing down-dip, top-to-the-north shear sense. The shear zone, the accretionary melange and the high- grade metamorphic rocks are cut by an undeformed granitoid with a 21 Ma Rb/Sr biotite age, analytically indistinguishable from the Rb/Sr biotite ages in the surrounding footwall gneisses. The estimated pressure of the metamorphism, and that of the granitoid emplacement, indicate that the high-grade metamorphic rocks were rapidly exhumed at ~ 24 Ma from a depth of ~ 14 km to ~ 7 km by activity along the shear zone. The subsequent exhumation of the metamorphic rocks to the surface occurred during Pliocene-Quaternary times in a transpressive ridge between two overstepping fault segments of the North Anatolian Fault zone. The high-grade metamorphic rocks of the Kazdag ˘ range are surrounded by voluminous calc-alkaline volcanic and plutonic rocks of Late Oligocene-Early Miocene age, which formed above the northward-dipping Hellenic subduction zone. The magmatic arc setting of the core complex and stratigraphic evidence for subdued topography in northwest Turkey prior to the onset of extension suggest that the latest Oligocene regional extension was primarily related to the roll-back of the subduction zone rather than to the gravitational collapse.

Cretaceous and Triassic subduction-accretion, high-pressure–low-temperature metamorphism, and continental growth in the Central Pontides, Turkey

Biostratigraphic, isotopic, and petrologic data from the Central Pontides document major southward growth of the Eurasian continental crust by subduction-accretion during the Cretaceous and Triassic Periods. A major part of the accreted material is represented by a crustal slice, 75 km long and up to 11 km thick, consisting of metabasite, metaophiolite, and mica schist that represent underplated Tethyan oceanic crustal and mantle rocks. They were metamorphosed at 490 degrees C and 17 kbar in mid-Cretaceous time (ca. 105 Ma). The syn-subduction exhumation occurred in a thrust sheet bounded by a greenschist facies shear zone with a normal sense of movement at the top and a thrust fault at the base. A flexural Foreland basin developed in front of the south-vergent high-pressure-low-temperature (HP-LT) metamorphic thrust sheet; the biostratigraphy of the foreland basin constrains the exhumation of the HP-LT rocks to the lbronian-Coniacian, similar to 20 m.y. after the HP-LT metamorphism, and similar to 25 m.y. before the terminal Paleocene continental collision. The Cretaceous subduction-accretion complex is tectonically overlain in the north by oceanic crustal rocks accreted to the southern margin of Eurasia during the latest Triassic-earliest Jurassic. The Triassic subduction-accretion complex is made up of metavolcanic rocks of ensimatic arc origin and has undergone a high pressure, greenschist facies metamorphism with growth of sodic amphibole. Most of the Central Pontides consists of accreted Phanerozoic oceanic crustal material, and hence is comparable to regions such as the Klamath Mountains in the northwestern United States or to the Altaids in Central Asia.

Chemical and isotopic variations along the superfast spreading East Pacific Rise from 6 to 30°S

Chemical data of 39 fresh basaltic glasses from the East Pacific Rise (EPR) between 6 and 30°S and Pb, Sr, and Nd isotopic compositions of 12 basalt glasses are presented. Major and trace element data indicate a wide compositional range, including primitive basalts (Mg#=0.67) and highly evolved FeTi-basalts (Mg#=0.34) [molMg/(Mg+Fe2+)]. The compositional range can be attributed to low-pressure fractional crystallization. Fractionation-corrected major element concentrations provide evidence for varying mantle melting conditions. Calculations of the melting conditions suggest melt generation in a rising upper mantle column between 20 and 10 kbar, at temperatures between 1430 and 1280°C, and total degrees of partial melting between 17 and 20% by weight. Leached and hand-picked basalt glasses display large variations in 87Sr/86Sr (0.70235–0.70270), 143Nd/144Nd (0.51312–0.51323), and 206Pb/204Pb (18.064–18.665), but are similar to other N-type MORB from the EPR. The isotopic ratios of basalts from 13 to 23°S show strong correlations and delineate two systematic trends. From 23 to 17°S, 87Sr/86Sr and Pb isotope ratios increase and 143Nd/144Nd decrease in agreement with previous results (Mahoney et al. 1989). A reverse trend is indicated by basalts from 17 to 13°S. However, K/Ti and (La/Sm)N continuously increase from 23 to 13°S. This opposite behavior indicates a recent decoupling of isotopic and minor element ratios in the mantle between 13 and 17°S. North of 13.5°S (Garrett Fracture Zone), isotopic data show no systematic variation with ridge location and display an overall weaker covariation. The results suggest that the isotopic variations and ridge segmentation appear to be unrelated and that major ridge offsets apparently coincide with changes in mantle melting conditions (P, T, F) (F, degrees of melting). There is no evidence for a systematic relationship between calculated melting conditions and second order ridge segmentation. Our isotopic data provide further evidence for regionally confined chemical variations in the mantle at 5 to 30°S. We interpret the isotopic trends as reflecting melting of distinct smallvolume and old enriched mantle components. In contrast, variations in trace elements are attributed to young mantle differentiation processes.

Nd–Sr–Pb isotopes of Tengchong Cenozoic volcanic rocks from western Yunnan, China: evidence for an enriched-mantle source

Abstract Tengchong Cenozoic volcanic activity in the southeastern Himalayan belt of western Yunnan is considered to be rift-related after the Indo-Eurasia collision. In this study, Nd–Sr–Pb isotopic compositions were analyzed in order to understand the genesis of the Cenozoic volcanic rocks. Five basalts and andesitic basalts are characterized by high 87Sr/86Sr ratios (0.7057–0.7081), low ϵNd values (−1.1 to −5.7), and particularly high 208Pb∗/206Pb∗ ratios (1.08–1.12). Twenty samples of rocks, which include granite, granodiorite, and amphibolite from the same region, were analyzed to evaluate the role of contamination and assimilation in the volcanic rocks. Granites have high 87Sr/86Sr ratios and low ϵNd values, indicating a crustal origin. Granodiorites have 87Sr/86Sr ratios of 0.7069–0.7100 and ϵNd-values of −2.9 to −7.6 that indicate contribution of a mantle component. Amphibolites that are characterized by high ϵNd-values of 9.4 to −1.7 may represent remnants of obducted oceanic crust of the Neo-Tethyan basin. The volcanic rocks are distinguished from the granitoids and amphibolites in terms of Pb isotopic compositions. This observation probably implies that crustal contamination played a minor role in the origin of the volcanic rocks. Isotopic evidence for the volcanic rocks indicates that they probably originated from melting of an enriched-mantle source, that is, ascribed to assimilation of subducted oceanic crust and sediments.

Provenance of the Beihuaiyang lower-grade metamorphic zone of the Dabie ultrahigh-pressure collisional orogen, China: evidence from zircon ages

The Beihuaiyang zone of the Dabie ultrahigh-pressure collisional belt, which was formed by the Early Mesozoic collision of the North and South China (Yangtze) Blocks, is diversely interpreted as an active or a passive sedimentary sequence. It comprises the Luzhenguan and Foziling complexes. Six granitoid rocks of the Luzhenguan complex are dated at 770 Ma to 720 Ma and one schist sample also contains detrital zircons of 760 Ma to 720 Ma. These data indicate that this complex was of the South China affinity and probably originated from the northern Yangtze Block. Two quartzite samples of the Foziling complex contain abundant detrital zircons of Archean to Paleozoic ages, clustering around 2.5 Ga, 1.9 ‐ 1.8 Ga, 1.0‐ 0.7 Ga, and 0.5 ‐ 0.4 Ga. This age spectrum demonstrates a mixed source of these two blocks. The youngest zircons suggest a Paleozoic deposition along an active continental margin south of the North China Block, while these Late Proterozoic zircons are characterized for the Yangtze Block. Subsequently, it is suggested that a terrain drifted from the northern Yangtze Block in Early Paleozoic must have been situated to the southern margin of the North China Block to provide material source prior to the final collision in Early Mesozoic. q 2003 Elsevier Ltd. All rights reserved.

Zircon U-Pb and Pb-isotope fractionation during stepwise HF acid leaching and geochronological implications

Abstract Two homogeneous zircons, zircon 91500 (Kuehl Lake, Ontario) and Phalaborwa zircon (South Africa), were analysed to study U and Pb behaviour during stepwise HF acid leaching. From the Phalaborwa zircon, ∼11% of radiogenic Pb and ∼7% of U were extracted into solution at the first leaching step and a reverse discordance was obtained. After extraction of ∼27.0% of the total radiogenic Pb budget and ∼24.5% of the U, the zircon residue gave concordant ages. It contained ∼75.8% of total radiogenic 206Pb budget, ∼76.4% of total radiogenic 207Pb budget, and ∼68.0% of total radiogenic 208Pb budget, indicating different mobility of Pb isotopes. The 208Pb*/206Pb* ratios of the solutions and the residue decrease systematically from ∼1.10 to ∼0.60. This pattern probably demonstrates that domains with different Th/U ratios were successively sampled during acid attack. In contrast to Phalaborwa zircon, only ∼1% of the total radiogenic Pb budget was extracted in leaching steps from each of three zircon 91500 fragments. Highly reverse discordant ages are observed in the first leaching step of the Phalaborwa zircon. This phenomenon is consistent with the recent studies of Mattinson [Partial dissolution analysis (PDF) of zircon: past, present, future. 11th Annual V. M. Goldschmidt Conference, Abstract volume, 3368.pdf. (2001)] and Davis and Krogh [Chem. Geol. 172 (2000) 41]. Due to the fractionations between U and Pb and between radiogenic Pb isotopes observed in the first leaching step, discordant domains or zonings of zircons have to be completely removed in the leaching steps to avoid any artificial high 207Pb*/206Pb* ratio in the residues. Evaluation of data from leaching studies shows that the extractability of Pb is positively correlated with the total radiogenic Pb budgets in zircons and subsequently with U contents and ages of zircons, indicating an important role of radiation damage. This correlation is consistent with the fact that old zircons with high U contents are much more susceptible to Pb loss than younger ones with low U contents.

Nd, Sr, and Pb isotopic evidence for diverse lithospheric mantle sources of East African Rift carbonatites

Abstract Carbonatites may provide valuable information on mantle source compositions as their isotopic ratios are insensitive to crustal contamination. In order to place constraints on mantle sources, nineteen samples from three Miocene to Quaternary carbonatite areas in the East African Rift were analysed for their Sr, Nd, and Pb isotopic compositions. The samples from Kerimasi (northern Tanzania), Homa Mountain, and Wasaki Peninsula (both Lake Victoria, Kenya) as a whole show considerable variations in their isotope ratios (0.70327–0.70502 for 87Sr/86Sr, 0.51249–0.51283 for 143Nd 144Nd, 18.72–20.41 for 206Pb/204Pb, 15.52–15.78 for 207Pb/204Pb, and 39.22–40.47 for 208Pb/204Pb) that lie between the inferred compositions for HIMU (high 238U/204Pb mantle) and EM I (enriched mantle I) components in most isotope plots. The internal isotopic variations of the three carbonatite areas define distinct arrays and diverse trends in isotope diagrams. Although the isotope data define linear arrays in Sr Nd and Pb Pb diagrams, which suggest binary mixing between HIMU and EM I mantle components, neither the isotopic compositions of the carbonatites as a whole nor the compositional ranges for individual carbonatite occurrences can be explained by such a process. This clearly emerges from the absence of linear data trends in Sr Pb and Nd Pb isotope plots and from the lack of consistent endmember compositions. These features are also displayed by previously published isotope data for East African carbonatites. It is therefore suggested that carbonatite complexes within the East African Rift have isotopically distinct and small mantle sources that are probably not adequately described in terms of the mantle components defined for oceanic basalts. Most likely, these sources are located in a heterogeneous lithospheric mantle and were produced by enrichment and depletion processes at different times and degrees.

An Oligocene ductile strike-slip shear zone: The Uludağ Massif, northwest Turkey—Implications for the westward translation of Anatolia

The Uludag Massif in northwest Turkey represents an exhumed segment of an Oligocene ductile strike-slip shear zone that is over 225 km long and has ~100 km of right-lateral strike-slip displacement. It forms a faultbounded mountain of amphibolite-facies gneiss and intrusive Oligocene granites. A shear-zone origin for the Uludag Massif is indicated by: (1) its location at the tip of the active Eskisehir oblique-slip fault, (2) pervasive subhorizontal mineral lineation in the gneisses with a right-lateral sense of slip, (3) foliation with a consistent strike, (4) the presence of a subvertical synkinematic intrusion, and (5) the alignment of the Eskisehir fault, synkinematic metagranite, and the strike of the foliation and mineral lineation. The shear zone nucleated in amphibolite-facies gneisses at peak pressure-temperature (P-T) conditions of 7.0 kbar and 670 °C, and it preserves Eocene (49 Ma) and Oligocene (36–30 Ma) Rb/Sr muscovite and biotite cooling ages. The shear zone was active during the latest Eocene and Oligocene (38–27 Ma), as shown by the crystallization and cooling ages from synkinematic granite. A 27 Ma postkinematic granite marks the termination of shear-zone activity. The 20–21 Ma apatite fi ssion-track (AFT) ages indicate rapid exhumation during the early Miocene. A 14 Ma AFT age from an Uludag gneiss clast deposited in a neighboring Neogene basin shows that the shear zone was on the surface by the late Miocene. Results of this study indicate that during the Oligocene, crustal-scale right-lateral strikeslip faults were transporting crustal fragments from Anatolia into the north-south– extending Aegean; this implies that the westward translation of Turkey, related to the Hellenic slab suction, started earlier than the Miocene Arabia-Eurasia collision.

Geochemical and isotopic constraints on the genesis of the Late Palaeozoic Deliktas and Sivrikaya granites from the Kastamonu granitoid belt (Central Pontides, Turkey)

The Late Palaeozoic Sivrikaya and Deliktas granitoids of the Kastamonu granitoid belt (KGB) are of sub-alkaline affi- nity, belong to the high-K calc-alkaline series and display features of transitional to S-type granites. Sivrikaya granitoid is host to biotite-hornblende granodiorite-tonalite and minor two-mica granites. The rocks are 303 - 300 Ma old, have low initial 87 Sr/ 86 Sr ratios (0.7041- 0.708), moderately low eNd(t) values (-1 to - 3.8) and young TDM model ages (0.75 to 1.08 Ga). All these charac- teristics, combined with low Al2O3/(FeO + MgO + TiO2) and (Na2O + K2O)/(FeO + MgO + TiO2) and δ 18 O values of 10-11.6 ‰ point to dehydration melting of heterogeneous protoliths dominated by amphibolite and greywackes-type sources with mantle contribution. Chondrite-normalized REE patterns of the Sivrikaya rocks are characterized by concave-upward patterns suggesting that amphibole played a more significant role than garnet during magma segregation. The main portion of the Deliktas granitoid consists of peraluminous muscovite-rich monzogranite. Compared to Sivrikaya, rocks from this pluton have higher initial Sr ra- tios (0.7109 - 0.7185), older Nd model ages (1.2 to 2.2 Ga) and similar eNd(t) values ( - 2.0 to - 4.7). U - Pb zircon analyses give an age range of 295 - 275 Ma. The nearly constant δ 18 O values (~ 11.5 to 11.7‰) in conjunction with the chemical characteristics in- dicate a predominantly pelitic source similar to the basement, which consists of felsic high-grade granulite-facies metasedimen- tary rocks, of continental origin.