Detrital chromites in metasediments of the East-Arabian continental margin in the Saih Hatat area: constraints for the palaeogeographic setting of the Hawasina and Semail basins (Oman Mountains)

 In Oman, the convergence between Arabia and Eurasia resulted in the Late Cretaceous overthrusting of oceanic crust and mantle lithosphere onto the Arabian continental margin. During this compressional event, a part of the continental plate was subducted to a depth of more than 60 km (0.5 GPa, 250–350  °C to more than 2.0 GPa, 550  °C) resulting in progressive metamorphism of the continental margin sediments, well exposed in the Saih Hatat tectonic window, northeastern Oman Mountains. We attempt to constrain the possibility of one continuous history of extension (starting along the east Arabian continental margin in the Permian) that was followed by one continuous history of convergence starting at 90 Ma near a dead oceanic ridge. This compression resulted in the observed progressive metamorphism by ophiolite overthrusting onto the continental margin. Constraining arguments are the palaeogeographic setting before ophiolite obduction of the As Sifah units and the Hawasina Complex near Ghurba. Detrital chromites in the Triassic–Cretaceous metasediments of the Hawasina Complex are compared with magmatic Semail chromites, and the whole-rock chemistry of these metasediments and associated metabasites are investigated. In contrast to former hypotheses, differences in the chemical composition between detrital and magmatic chromites, and the probable origin of all detrital chromites in the Hawasina Basin from Permian age oceanic rocks, suggest that the high-pressure metamorphic sediments of As Sifah can be considered as part of the basal deposits of the Hawasina Basin. Received: 1 September 1998 / Accepted: 18 January 1999     

The upper Hawasina nappes in the central Oman Mountains: stratigraphy,palinspastics and sequence of nappe emplacement

Abstract

In the Oman mountains, a succession of sedimentary decollement nappes, the Hawasina nappes, is sandwiched between the Samail ophiolite nappe and its underlying melange and the “autochthonous” sequences of the Arabian platform. The sediments of the Hawasina nappes document the Mesozoic evolution of the northeastern Arabian continental margin and the adjacent Tethys Ocean. In earlier paleogeographic reconstructions, based on simple telescoping of the tectonic units, the upper Hawasina nappes represent the distal part and the lower nappes the proximal part of the margin. New stratigraphic data suggest a revision of the paleogeography and a more complex model for nappe emplacement in the central Oman mountains. The lower Hawasina nappes with their Jurassic and Cretaceous base of slope and basin sediments (Hamrat Duru, Wahrah) form the original cover of part of the upper Hawasina nappes. In the latter (Al Ayn, Haliw), Triassic pelagic sediments, locally overlain by massive sandstone successions are preserved. Complete Mesozoic sequences with pelagic Cenomanian sediments as youngest dated elements are found in the highest Hawasina units (Al Aridh and Oman Exotics). The stratigraphic data indicate polyphase thrusting in the central Oman mountains. Downward propagation of thrusting in front of the Samail is responsible for cutting the original stratigraphie sequence into a number of thrust-sheets, involving successively older and more external formations. This kind of thrust propagation eventually leads to the observed superposition of originally lower stratigraphie units onto their original cover. Regional deformation of the nappe contacts in post-nappe culminations (J. Akhdar, Saih Hatat) is related to ramp-flat-systems in the Arabian foreland.     

西秦岭月照 琵琶寺地区火山岩地球化学特征及地质意义

月照琵琶寺地区的变质火山岩出露于甘肃武都县境内,夹在泥盆系和前震旦碧口群地层之间。按地球化学分类,火山岩可划分为双峰式和碱性两个系列。双峰式火山岩系由低K富Na拉斑玄武岩和酸性英安岩、流纹岩组成。该玄武岩具有类似MORB的微量元素特征,平坦的稀土配分模式,但富Th贫Nb显示其受到陆壳混染的影响。该套火山岩总体特征指示其为大陆裂谷向成熟洋盆转化阶段的产物,从而推测本区火山岩可能为勉略古洋盆西延的分支产物。     

峨眉山大火成岩省底部枕状玄武岩的地质地球化学特征及成因探讨

云南上沧二叠纪枕状玄武岩位于峨眉山大火成岩省内带底部。该玄武岩全岩SiO₂含量为49.36%～51.36%,TiO₂含量为1.28%～1.91%,MgO含量为6.08%～10.79%,Mg^#值为53～69,Al₂O₃含量为13.91%～16.31%,Na₂O含量为1.59%～3.72%,K₂O含量较低,为0.18%～0.76%,并且Na₂O/K₂O比值较高,为4.27～10.48,属拉斑系列玄武岩。具有右倾斜型稀土元素配分模式,重稀土和部分高场强元素与N-MORB接近,轻稀土和部分大离子亲石元素介于N-MORB和上地壳之间。(La/Sm)_N比值大多数低于2.0(1.4～2.2),(Ce/Yb)_N比值介于2.2～3.4之间,明显低于OIB,不具有OIB的地球化学特征,在Th/Hf-Ta/Hf构造环境判别图解中位于陆缘裂谷玄武岩区域,推测上沧枕状熔岩是古特提斯洋俯冲到扬子板块边缘形成的初始裂谷扩张的产物。     

Accreted fragments of the Late Cretaceous Caribbean–Colombian Plateau in Ecuador

The eastern part of the Western Cordillera of Ecuador includes fragments of an Early Cretaceous (≈123 Ma) oceanic plateau accreted around 85–80 Ma (San Juan–unit). West of this unit and in fault contact with it, another oceanic plateau sequence (Guaranda unit) is marked by the occurrence of picrites, ankaramites, basalts, dolerites and shallow level gabbros. A comparable unit is also exposed in northwestern coastal Ecuador (Pedernales unit).

Picrites have LREE-depleted patterns, high Nd_i and very low Pb isotopic ratios, suggesting that they were derived from an extremely depleted source. In contrast, the ankaramites and Mg-rich basalts are LREE-enriched and have radiogenic Pb isotopic compositions similar to the Galápagos HIMU component; their Nd_i are slightly lower than those of the picrites. Basalts, dolerites and gabbros differ from the picrites and ankaramites by flat rare earth element (REE) patterns and lower Nd; their Pb isotopic compositions are intermediate between those of the picrites and ankaramites. The ankaramites, Mg-rich basalts, and picrites differ from the lavas from the San Juan–Multitud Unit by higher Pb ratios and lower Nd_i.

The Ecuadorian and Gorgona 88–86 Ma picrites are geochemically similar. The Ecuadorian ankaramites and Mg-rich basalts share with the 92–86 Ma Mg-rich basalts of the Caribbean–Colombian Oceanic Plateau (CCOP) similar trace element and Nd and Pb isotopic chemistry. This suggests that the Pedernales and Guaranda units belong to the Late Cretaceous CCOP. The geochemical diversity of the Guaranda and Pedernales rocks illustrates the heterogeneity of the CCOP plume source and suggests a multi-stage model for the emplacement of these rocks. Stratigraphic and geological relations strongly suggest that the Guaranda unit was accreted in the late Maastrichtian (≈68–65 Ma).     

Sr, Nd and Pb Isotopic Systematics of the Cenozoic Basalts of the Korean Peninsula and Their Implications for the Permo-Triassic Continental Collision Boundary

Sr, Nd and Pb isotopic compositions of the Cenozoic basalts were analyzed from Baengnyeongdo Island, Jeongok, Ganseong, and Jejudo Island of Korea. They reveal relatively enriched Sr and Nd isotopic compositions (⁸⁷Sr/⁸⁶Sr = 0.703300.70555, ¹⁴³Nd/¹⁴⁴Nd = 0.512980.51256) compared with MORB.²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb values of all the analyzed Korean basalts lie above the Northern Hemisphere Reference Line (NHRL) defined by Hart (1984). Pb isotopic compositions of basalts from Jejudo Islands (²⁰⁶Pb/²⁰⁴Pb = 18.6119.12, ²⁰⁷Pb/²⁰⁴Pb = 15.5415.69, ²⁰⁸Pb/²⁰⁴Pb = 38.9839.72) are significantly more radiogenic than the rest (²⁰⁶Pb/²⁰⁴Pb = 17.7218.03, ²⁰⁷Pb/²⁰⁴Pb = 15.4415.58, ²⁰⁸Pb/²⁰⁴Pb = 37.7738.64). The Cenozoic Korean basalts thus can be divided into two groups based on their Sr, Nd and Pb isotopic compositions. The north group reveals mixing between DMM and EM1 while the south group displays DMM-EM2 mixing. Such a distribution is the same as Chinese Cenozoic basalts and it can be interpreted that the subcontinental lithospheric mantle under Korea represents simple lateral continuation of the South and North China Blocks. We suggest that Korean continental collision zone cross the Korean Peninsula through the region between the north and south basalt groups of Korea.     

西秦岭月照-琵琶寺地区火山岩地球化学特征及地质意义

月照-琵琶寺地区的变质火山岩出露于甘肃武都县境内,夹在泥盆系和前震旦碧口群地层之间.按地球化学分类,火山岩可划分为双峰式和碱性两个系列.双峰式火山岩系由低K富Na拉斑玄武岩和酸性英安岩、流纹岩组成.该玄武岩具有类似MORB的微量元素特征,平坦的稀土配分模式,但富Th贫Nb显示其受到陆壳混染的影响.该套火山岩总体特征指示其为大陆裂谷向成熟洋盆转化阶段的产物,从而推测本区火山岩可能为勉略古洋盆西延的分支产物.     

The nature of the sub-continental mantle: constraints from the major-element composition of continental flood basalts

Many continental flood basalts (CFB) have isotope and trace-element signatures that differ from those of oceanic basalts and much interest concerns the extent to which these reflect differences in their upper mantle source regions. A review of selected data sets from the Mesozoic and Tertiary CFB confirms significant differences in their major- and trace-element compositions compared with those of basalts erupted through oceanic lithosphere. In general, those CFB suites characterised by low Nb/La, high (⁸⁷Sr/⁸⁶Sr)_i and low εNd_i tend to exhibit relatively low TiO₂, CaO/Al₂O₃, Na₂O and/or Fe₂O₃, and relatively high SiO₂. In contrast, those which have high Nb/La, low (⁸⁷Sr/⁸⁶Sr)_i and high εNd_i ratios, like the upper units in the Deccan Traps, have major- and trace-element compositions similar to oceanic basalts. It would appear that those CFB that have distinctive isotope and trace-element ratios also exhibit distinctive major-element contents, suggesting that major and trace elements have not been decoupled significantly during magma generation and differentiation.

When compared (at 8% MgO) with oceanic basalt trends, the displacement of many CFB to lower Na₂O, Fe₂O₃^*, TiO₂ and CaO/Al₂O₃, but higher SiO₂, at similar Mg#, is not readily explicable by crustal contamination. Rather, it reflects source composition and/or the effects of the melting processes. The model compositions of melts produced by decompression of mantle plumes beneath continental lithosphere have relatively low SiO₂ and high Fe₂O₃^*. In contrast, the available experimental data indicate that partial melts of peridotite have low TiO₂, Na₂O and Fe₂O₃^*CaO/Al₂O₃, if the peridotite has been previously depleted by melt extraction. Moreover, melting of hydrated, depleted peridotite yields SiO₂-rich, Fe₂O₃- and CaO-poor melts. Since anhydrous, depleted peridotite has a high-temperature solidus, it is argued that the source of these CFB was variably melt depleted and hydrated mantle, inferred to be within the lithosphere. Isotope data suggest these source regions were often old and relatively enriched in incompatible trace elements, and it is envisaged that H₂O±CO₂ were added at the same time as the incompatible elements. An implication is that a significant proportion of the new continental crust generated since the Permian reflected multistage processes involving mobilization of continental mantle lithosphere that was enriched in minor and trace elements during the Proterozoic.     

Melting of the subcontinental lithospheric mantle by the Emeishan mantle plume; evidence from the basal alkaline basalts in Dongchuan, Yunnan, Southwestern China

The Emeishan continental flood basalt (ECFB) sequence in Dongchuan, SW China comprises a basal tephrite unit overlain by an upper tholeiitic basalt unit. The upper basalts have high TiO₂ contents (3.2–5.2 wt.%), relatively high rare-earth element (REE) concentrations (40 to 60 ppm La, 12.5 to 16.5 ppm Sm, and 3 to 4 ppm Yb), moderate Zr/Nb and Nb/La ratios (9.3–10.2 and 0.6–0.9, respectively) and relatively high _{Nd (t)} values, ranging from − 0.94 to 2.3, and are comparable to the high-Ti ECFB elsewhere. The tephrites have relatively high P₂O₅ (1.3–2.0 wt.%), low REE concentrations (e.g., 17 to 23 ppm La, 4 to 5.3 ppm Sm, and 2 to 3 ppm Yb), high Nb/La (2.0–3.9) ratios, low Zr/Nb ratios (2.3–4.2), and extremely low _{Nd (t)} values (mostly ranging from − 10.6 to − 11.1). The distinct compositional differences between the tephrites and the overlying tholeiitic basalts cannot be explained by either fractional crystallization or crustal contamination of a common parental magma. The tholeiitic basalts formed by partial melting of the Emeishan plume head at a depth where garnet was stable, perhaps > 80 km. We propose that the tephrites were derived from magmas formed when the base of the previously metasomatized, volatile-mineral bearing subcontinental lithospheric mantle was heated by the upwelling mantle plume.     

Geochemical characteristics of lavas from Broken Ridge, the Naturaliste Plateau and southernmost Kerguelen Plateau: Cretaceous plateau volcanism in the southeast Indian Ocean

Lavas from several major bathymetric highs in the eastern Indian Ocean that are likely to have formed as Early to Middle Cretaceous manifestations of the Kerguelen hotspot are predominantly tholeiitic; so too are glass shards from Eocene to Paleocene volcanic ash layers on Broken Ridge, which are believed to have come from eruptions on the Ninetyeast Ridge. The early dominance of tholeiitic compositions contrasts with the more recent intraplate, alkalic volcanism of the Kerguelen Archipelago. Isotopic and incompatible-element ratios of the plateau lavas are distinct from those of Indian mid-ocean ridge basalts; their Nd, Sr, ²⁰⁷Pb/²⁰⁴Pb and isotopic ratios overlap with but cover a much wider range than measured for more recent oceanic products of the Kerguelen hotspot (including the Ninetyeast Ridge) or, indeed, oceanic lavas from any other hotspot in the world. Samples from the Naturaliste Plateau and ODP Site 738 on the southern tip of the Kerguelen Plateau are particularly noteworthy, with ε_Nd(T) = − 13 to −7, (⁸⁷Sr/⁸⁶Sr)_T=0.7090 to 0.7130 and high ²⁰⁷Pb/²⁰⁴Pb relative to ²⁰⁶Pb/²⁰⁴Pb. In addition, the low-ε_Nd(T) Naturaliste Plateau samples are elevated in SiO₂ (> 54 wt%). In contrast to “DUPAL” oceanic islands such as the Kerguelen Archipelago, Pitcairn and Tristan da Cunha, the plateau lavas with extreme isotopic characteristics also have relative depletions in Nb and Ta (e.g., Th/Ta, La Nb > primitive mantle values); the lowest ε_Nd(T) and highest Th/Ta and La Nb values occur at sites located closest to rifted continental margins. Accepting a Kerguelen plume origin for the plateau lavas, these characteristics probably reflect the shallow-level incorporation of continental lithosphere in either the head of the early Kerguelen plume or in plume-derived magmas, and suggest that the influence of such material diminished after the period of plateau construction. Contamination of asthenosphere with the type of material affecting Naturaliste Plateau and Site 738 magmatism appears unlikely to be the cause of low-²⁰⁶Pb/⁰⁴Pb Indian mid-ocean ridge basalts. Finally, because isotopic data for the plateaus do not cluster or form converging arrays in isotope-ratio plots, they provide no evidence for either a quickly evolving, positive ε_Nd, relatively high-²⁰⁶Pb/²⁰⁴Pb plume composition, or a plume source dominated by mantle with ε_Nd of −3 to 0.     

浙江建德新生代玄武岩源区特征及其成因

中国东南部浙江境内分布有大量新生代板内玄武岩，这些玄武岩的分布受到三条北东—南西向的断裂控制，将浙江玄武岩分为西部、中部、东部三个区域。其中，出露于浙江西部江山-绍兴断裂带的建德玄武岩是浙江境内最古老的新生代玄武岩（约40 Ma）。为进一步认识浙江境内新生代岩浆作用的本质，测定了建德玄武岩的元素组成和Sr、Nd、Hf、Pb同位素组成，并在与浙江境内其它新生代玄武岩对比的基础上，探讨它们之间的成因联系。建德玄武岩为碧玄岩，与浙江西部其它新生代玄武岩一样，碱性程度明显高于浙江中部和东部的新生代玄武岩（以弱碱性的碱性橄榄玄武岩和拉斑玄武岩为主）。这些玄武岩具有较低的SiO₂ (41.3~42.3 wt%）和Al₂O₃（9.70~12.6 wt%）含量，较高的MgO（8.90~15.6 wt%）、CaO（8.92~12.1 wt%）、TiO₂（2.78~3.18 wt%）和Fe₂O₃^T（14.1~16.2 wt%）含量以及较高的Ca/Al（1.02~1.16）比值。不相容微量元素组成上与火成碳酸岩具有亲缘性，即蛛网图上表现为明显的K、Zr、Hf、Ti的负异常（Hf/Hf*=0.74~0.77, Ti/Ti*=0.70~0.74），同时具有高的Zr/Hf比值（48.5~50.1），表明其地幔源区含有碳酸盐组分。建德玄武岩具有亏损的Sr-Nd-Hf同位素组成（⁸⁷Sr/⁸⁶Sr=0.7032~0.7034, εNd=5.85~5.95, εHf=7.78~8.56）和较高的²⁰⁶Pb/²⁰⁴Pb（18.491~18.554）、²⁰⁷Pb/²⁰⁴Pb（15.488~15.518）和²⁰⁸Pb/²⁰⁴Pb（38.387~38.523）比值。相比浙江中部和东部玄武岩，浙江西部玄武岩及建德玄武岩具有更高的碱含量、不相容微量元素含量、La/Yb比值和更明显的K、Zr、Hf、Ti负异常，表明浙江西部玄武岩是碳酸盐化地幔低程度熔融的产物。浙江新生代玄武岩的Ti/Ti*与Ba/Th、K/La比值之间较好的正相关性表明其源区存在两端元混合的特征。其中，以浙江西部玄武岩（包括建德玄武岩）为代表的地幔端元是由含碳酸盐的再循环洋壳熔融产生的碳酸盐熔体与亏损地幔反应形成的碳酸盐化地幔，以较低的Ba/Th、K/La和Ti/Ti*比值为特征。以浙江东部玄武岩为代表的地幔端元具有和碳酸盐化地幔端元互补的、较高的Ba/Th、K/La和Ti/Ti*比值，代表熔融残余的再循环洋壳。因此，浙江新生代玄武岩的地幔源区是不均一的，这种不均一性主要是由具有成因联系的两种端元组分所控制。     

Metamorphic response within different subduction–obduction settings preserved on the NE Arabian margin

Metamorphic rocks form a minor component of the NE Arabian margin in Oman and the United Arab Emirates (UAE). Conditions span almost the entire range of crustal metamorphism from very high-P/low-T eclogite and blueschist to high-P/moderate-T epidote- to upper-amphibolite and low-P/high-T granulite facies. The NE Arabian margin experienced at least six metamorphic events, each characterized by distinct peak metamorphic temperature, depth of burial, average thermal gradient and timing. Synthesis of the available metamorphic data defines five different tectonic settings that evolved during the middle Cretaceous: [1] The Saih Hatat window exposes former continental margin crust that was buried and metamorphosed in a SW-dipping subduction system. Lower-plate units in the window include relict oceanic crust with eclogite (M1–M2) parageneses that recrystallized at pressures of ~14–23 kbar under very low thermal gradients of 7–10 °C/km. Peak metamorphism occurred at ~110 Ma. Peak assemblages were overprinted by garnet–glaucophane-blueschist foliations (M3) at about ~104–94 Ma that formed at ~10–15 kbar and 10–15 °C/km during the first-stage of isothermal exhumation. [2] Metamorphic soles in the footwall of the Semail ophiolite experienced a two-stage history of deep burial and peak metamorphism at ~96–94 Ma, followed by retrogression during obduction onto the continental margin between ~93 and 84 Ma. Peak metamorphic garnet–clinopyroxene–hornblende–plagioclase assemblages (M4s), exposed at highest structural levels, formed at 743 ± 13 °C and 10.7 ± 0.4 kbar, indicating Barrovian thermal regimes of 20.0 ± 2.2 °C/km. Burial of seafloor sediments and oceanic crust to ~38 km depth, was attained within a short-lived, NE-dipping intra-oceanic subduction system. The relatively high average thermal gradient during the peak of metamorphism was the result of heating after subcretion onto the base of hanging-wall oceanic lithosphere. [3] The Bani Hamid terrane consists of seafloor cherts and calcareous turbidites, metamorphosed to low-P/high-T granulite condition at ~96–94 Ma. Diagnostic assemblages (M4b) such as orthopyroxene–cordierite–quartz–plagioclase and orthopyroxene–sapphirine–hercynite–quartz–plagioclase, formed at conditions averaging ~915 ± 35 °C, ~6.1 ± 0.9 kbar and ~42.9 ± 6.5 °C/km. The elevated average thermal gradient, combined with significant depths of burial, is anomalous for typical oceanic settings. This suggests that these sea-floor sediments were buried to ~22 km depths within the intra-oceanic subduction system, accreted onto the hanging-wall, and metamorphosed at high-T during subduction of a recently active spreading ridge. [4] A plausible plate tectonic arrangement that can account for the different metamorphic elements on the Arabian margin is one composed of divergent subduction systems: a relatively long-lived SW-dipping subduction zone at the continental margin, and a short-lived, NE-dipping intra-oceanic subduction system. Consumption of the intervening oceanic crust led to obduction of the Semail ophiolite and accreted metamorphic soles from the upper-plate of the floundered outboard subduction system. SW-directed obduction was initiated between 93.7 and 93.2 Ma and continued until ~84 Ma, producing lower-amphibolite to sub-greenschist facies retrograde fabrics in the metamorphic soles (M5) and sub-metamorphic melange in the footwall. [5] The lower-plate of the Saih Hatat window was reworked by top-to-NE extensional shear at epidote-greenschist facies grades (M6) between ~84 and 76 Ma. Crustal-scale structures were reactivated as extensional detachments that telescoped the continental margin, leading to isothermal decompression and development of an asymmetric core complex that segmented the Semail ophiolite and formed the Saih Hatat domal window.     

Mantle sources and crustal input as recorded in high-Mg Deccan Traps basalts of Gujarat (India)

Near-primitive picritic basalts in the northwestern Deccan Traps have MgO > 10 wt.% and consist of two groups (low-Ti and high-Ti) with markedly different incompatible element and Nd–Sr–Pb isotope characteristics. Many elemental characteristics of the low-Ti picritic basalts are similar to those of transitional or normal ocean ridge basalts. However, values of ratios like Ba/Nb (13–30) and Ce/Pb (4–11), and isotopic ratios (e.g., ε_Nd(t) + 0.3 to − 6.3, (²⁰⁷Pb/²⁰⁴Pb)_t 15.63–15.75 at (²⁰⁶Pb/²⁰⁴Pb)_t 18.19–18.84, δ¹⁸O_olivine as high as + 6.2‰) are far-removed from ocean-ridge-type values, indicating a significant contribution from continental crust. The crustal signature could represent crustal contamination of ascending magmas; alternatively, it could represent a minor component within the Indian lithospheric mantle of anciently subducted sedimentary material or fluids derived from subducted material. In contrast, the high-Ti picritic basalts are chemically and isotopically rather similar to recent shield lavas of the Réunion hotspot (e.g., ε_Nd(t) + 2 to + 4) and to volcanic rocks along the postulated pre-Deccan track of this hotspot in Pakistan. Neither type of picritic basalt is parental to the voluminous flows comprising the bulk of the Deccan Traps. However, many of the Deccan primary magmas could have been derived from mixtures of a high-Ti-type, Réunion-like source component and a component more similar to, or even more incompatible-element-depleted than, average ocean-ridge mantle.     

Pb and Nd isotopic composition of the Jigongshan granite: constraints on crustal structure of Tongbaishan in the middle part of the Qinling–Tongbai–Dabie orogenic belt, Central China

The Qinling–Dabie–Sulu belt is the world's largest ultrahigh pressure (UHP) metamorphic belt. The UHP metamorphism is well dated at 220–245 Ma in the Dabie–Sulu belt but at 507 Ma in the Qinling belt. The Tongbaishan is located between the Qinling orogenic belt to the west and the Dabie–Sulu UHP metamorphic belt to the east. It is the key area for studying the tectonic relation between the Qinling and Dabie–Sulu belts and the diachronous UHP metamorphism. The Jigongshan granitic pluton (t=128 Ma) with a total area of 1200 km², composed of monzogranite, was mostly emplaced into the Tongbai complex, an exposed basement in the Tongbaishan. The Jigongshan granites have SiO₂=69.85–72.35%, K₂O/Na₂O=0.87–1.13, A/CNK=0.91–1.03, Rb/Sr=0.14–0.25 and Th/U=3.3–12. Their REE compositions show strongly fractionated patterns with (La/Yb)_N=14–58 and Eu*/Eu=0.79–1.05. The granites are characterized by low radiogenic Pb isotopic composition. The present-day whole-rock Pb isotopic ratios are ²⁰⁶Pb/²⁰⁴Pb=16.707–17.055, ²⁰⁷Pb/²⁰⁴Pb=15.239–15.326 and ²⁰⁸Pb/²⁰⁴Pb=37.587–37.853, which are similar to that of the continental lower crust. Their _Nd(t) values range from −16 to −20, and depleted-mantle Nd model ages (T_DM) from 1.8 to 2.2 Ga. The above evidence indicates that the magma of the Jigongshan granites was derived from the partial melting of the continental crust. The Pb and Nd isotopic compositions of the Jigongshan granites resemble those of the Dabie core complex in the Dabieshan but are distinct from those of the Tongbai complex in the Tongbaishan. Thus, the Dabie core complex would be the magma source of the Jigongshan granites. The result implies that the Dabie core complex is extended to the west and constitutes the unexposed basement underlaying the Tongbai complex in the Tongbaishan.     

Isotopic (O, Sr, Nd) and trace element geochemistry of the Laouni layered intrusions (Pan-African belt, Hoggar, Algeria): evidence for post-collisional continental tholeiitic magmas variably contaminated by continental crust

The three layered intrusions studied in the Laouni area have been emplaced within syn-kinematic Pan-African granites and older metamorphic rocks. They have crystallized at the end of the regional high-temperature metamorphism, but are free from metamorphic recrystallization, revealing a post-collisional character. The cumulate piles can be interpreted in terms of two magmatic liquid lines of descent: one is tholeiitic and marked by plagioclase–olivine–clinopyroxene cumulates (troctolites or olivine bearing gabbros), while the other is calc-alkaline and produced orthopyroxene–plagioclase rich cumulates (norites). One intrusion (WL (West Laouni)-troctolitic massif), shows a Lower Banded Zone where olivine-chromite orthocumulates are interlayered with orthopyroxene-rich and olivine–plagioclase–clinopyroxene cumulates, whereas the Upper Massive Zone consists mainly of troctolitic and gabbroic cumulates. The other two massifs are more homogeneous: the WL-noritic massif has a calc-alkaline differentiation trend whereas the EL (East Laouni)–troctolitic massif has a tholeiitic one. Separated pyroxene and plagioclase display similar incompatible trace element patterns, regardless of the cumulate type. Calculated liquids in equilibrium with the two pyroxenes for both noritic and troctolitic cumulates are characterized by negative Nb, Ta, Zr and Hf anomalies and light REE enrichment inherited from the parental magmas. Troctolitic cumulates have mantle-derived δ¹⁸O (+5 to +6‰), initial ⁸⁷Sr/⁸⁶Sr (Sr_i=0.7030 to 0.7054), _Nd (+5 to −1) values whereas noritic cumulates are variably enriched in δ¹⁸O (+7 to +9‰), show negative _Nd (−7 to −12) and slightly higher Sr_i (0.7040–0.7065). Based on field, isotopic ratios are interpreted as resulting from a depleted mantle source (Sr_i=0.7030; _Nd=+5.1; δ¹⁸O=+5.1‰) having experience short term incompatible element enrichment and variable crustal contamination. The mantle magma was slightly contaminated by an Archaean lower crust in troctolitic cumulates, more strongly and with an additional contamination by an Eburnian upper crust in noritic cumulates. Lower crust input is recorded mainly by Sr and Nd isotopes and upper crust input by O isotopes. This is probably due to the different water/rock ratios of these two crust types. Assimilation of low amounts (<10%) of quartz-bearing felsic rocks, coming from both lower and upper crust, can explain the rise of SiO₂ activity, the enrichment in ¹⁸O and ⁸⁷Sr and the lowering of _Nd in the noritic cumulates compared to troctolitic ones. The geodynamic model proposed to account for the Laouni tholeiitic magmatism involves a late Pan-African asthenospheric rise due to a rapid lithospheric thinning associated with functioning of shear zones, which allowed tholeiitic magmas to reach high crustal levels while experiencing decreasing degrees of crustal contamination with time.     

大别山低级变质岩的构造背景

大别山南北两侧的浅变质岩是碰撞造山以前洋壳俯冲造山阶段的重要组成部分。木兰山片岩或张八岭群是俯冲的洋壳;苏家河群、信阳群和佛子岭群是由洋壳俯冲形成的海沟沉积,并因俯冲过程中的前进变形而形成增生楔;杨山煤系和梅山群是石炭纪弧前盆地沉积,并因俯冲过程中的前进变形而被增生楔逆掩。宿松群是扬子大陆被动边缘沉积,不是俯冲造山带的成员。因洋壳俯冲形成的弧和弧后盆地可能已被新生界沉积物掩盖。高压—超高压变质带是碰撞造山后期从深部折返的外来体。高压—超高压变质带正好处于洋壳和增生楔之间,破坏了早期洋壳俯冲造山带的完整性,使得洋壳俯冲造山阶段的特征被破坏,因而不易辨别。俯冲造山阶段应为奥陶纪到泥盆纪,碰撞造山阶段应从二叠纪开始。     

The Oman Exotics: a key to the understanding of the Neotethyan geodynamic evolution

Abstract

The study of the exotic blocks of the Hawasina Nappes (Sultanate of Oman) leads to give apposit data that allow us to propose a new paleogeographic evolution of the Oman margin in time and space. A revised classification of exotic blocks into different paleogeographical units is presented. Two newly introduced stratigraphic groups, the Ramaq Group (Ordovician to Triassic) and the Al Buda’ah Group (upper Permian to Jurassic) are interpreted as tilted blocks related to the Oman continental margin. The Kawr Group (middle Triassic to Cretaceous) is redefined and interpreted as an atoll-type seamount. The paleogeography and paleoenvironments of these units are integrated into a new scheme of the Neotethyan rifting history. Brecciae and olistoliths of the Hawasina series are interpreted to have originated from tectonic movements affecting the Oman margin and the Neotethyan ocean floor. The breccias of late Permian age were generated by the extension processes affecting the margin, and by the creation of the Neotethyan oceanic floor. The breccias of mid-late Triassic age coincide in time with the collision of the Cimmerian continents with Eurasia. In constrast, the breccias of late Jurassic and Cretaceous age are interpreted as resulting to the creation of a new oceanic crust (Semail) off the Oman margin.     

Geochemistry of mafic magmas from the Ungava orogen, Québec, Canada, and implications for mantle reservoir compositions at 2.0 Ga

The Ungava orogen of northern Québec is one of the best preserved Proterozoic mobile belts of the world, recording > 200 Ma of plate divergence and convergence. Voluminous magmatism associated with rifting of the Superior Province basement ≈2.04 Ga resulted in the development of a volcanic rift margin sequence and an ocean basin. Four distinct mafic magma suites were erupted: (1) continental basalts (Eskimo Formation, western and central Povungnituk Group) with moderate to high Zr/Nb and negative Nb anomalies which have interacted with the continental crust (ε_Nd(2.0 Ga)) from −7.4); (2) mafic lavas from the Flaherty Formation, eastern Povungnituk Group and some Watts Group lavas associated with passive margin rifting, having slightly enriched isotopic signatures (ε_{Nd(2.0 Ga)} = +2.7 to +4.4) compared to the contemporaneous depleted mantle, high (Nb/Y)_n and low Zr/Nb ratios (≈4.4 and ≈8.9, respectively); (3) a highly alkaline OIB-like suite (ε_{Nd(2.0 Ga)} = +2.3 to +3.2, (Nb/Y)_n> 12) within the Povungnituk Group composed of nephelinites, basanites and phonolites; and (4) depleted Mg-rich basalts and komatiitic basalts (ε_{Nd(2.0 Ga)} ≈ + 4.5 to + 5.5) with trace-element characteristics of N-MORB, but with higher Fe and lower Al than primitive MORB (Chukotat Group, Ottawa Islands and some Watts Group samples). The ocean basin into which these lavas were erupted was subsequently destroyed during subduction between ≈1.90 and ≈1.83 Ga, resulting in the development a magmatic arc (Narsajuaq terrane and Parent Group).

The Ungava magmas provide a unique window into the mantle at 2.0 Ga. The chemical and isotopic similarity of these Proterozoic magmas to modern-day magmas provides strong evidence that the interplay between depleted mantle, OIB mantle and sub-continental mantle during the Proterozoic was comparable to that of the modern Earth.     

Basalt geochemistry as a diagnostic indicator of tectonic setting

Basalt geochemistry can be used as a diagnostic indicator for determining the tectonic setting of origin, because specific plate tectonic settings often impart distinctive geochemical characteristics. For example: (1) mid-ocean ridge basalts (MORB) and oceanic island basalts (OIB) have clearly distinguishable trace element and Sr-Nd isotope geochemical characteristics; (2) arc related basalts, including IOAB (intra-oceanic arc basalts), IAB (island arc basalts) and CAB (continental arc basalts), exhibit following distinguishing features: all are characterized by low Nb/La ratios (<0.85) and negative Nb, Ta and Ti anomalies; most exhibit low Nb concentrations (<8 ppm), high positive ɛ_Nd values and low enrichment of incompatible elements except the continental arc shoshonitic basalts that possess high concentrations of incompatible trace elements and lower to negative ɛ_Nd values; (3) although contamination by continental crust or lithosphere can impart subduction-like signature (e.g., low Nb, low Ta and low Ti) and lead to misidentification of contaminated continental intraplate basalts as arc related, there are still some essential differences between continental intraplate basalts and arc related ones; such as: uncontaminated continental intraplate basalts have high Nb concentrations, Nb/La > 1, “hump-shaped” OIB-like trace element patterns and moderate positive ɛ_Nd values that distinguish them from the arc related ones; whereas, the contaminated continental intraplate basalts are characterized by pronounced negative Nb, Ta and Ti anomalies, but their concentrations of incompatible trace elements are conspicuously higher than those of subduction-zone basalts that also distinguishes them from the arc related ones; (4) an important difference between back-arc basin basalts (BABB) and the MORB is that the former exhibit both MORB-like and arc-like geochemical characteristics; (5) most oceanic plateau basalts (OPB) show diagnostic geochemical characteristics of enriched MORB (E-MORB) to transitional MORB (T-MORB); only the Kerguelen Plateau is an exception; the early (pre 90 Ma) volcanism of the Kerguelen Plateau is associated with the Early Cretaceous break-up of Gondwana and displays features of continental flood basaltic volcanism; with time, the tectonic setting of the Kerguelen plume-derived volcanism changed from a rifted continental margin setting (133–118 Ma) through a young, widening ocean (118–40 Ma), finally to an oceanic intraplate setting (~40 Ma to the present).Tectonic discrimination diagrams should not be used in isolation, but can still be useful as part of holistic geochemical characterization. For example: (1) MORB and OIB are distinguishable from each other in the 3Tb-Th-2Ta diagram; (2) the arc related basalts, including IOAB, IAB and CAB, constantly plot in the arc-related basalts fields in the Th/Yb-Ta/Yb diagram; (3) the 3Tb-Th-2Ta diagram can be utilized to fully illustrate both MORB-like and arc-like characteristics of BABB; (4) some discriminant diagrams (such as Zr/Y-Zr, Th/Yb-Ta/Yb, 3Tb-Th-2Ta and Hf/3-Th-Nb/16 diagrams) can be used to distinguish continental intra plate basalts from arc related ones; (5) although there are not any discrimination diagrams published that delineate an OPB field, some trace element diagrams can still reveal diagnostic characteristics of the OPB.     

Tectonic observations in the northern Saih Hatat,Sultanate of Oman

The Saih Hatat region, in northeastern Oman, is characterized by a large tectonic window, tectonically overlain during the upper Cretaceous by nappes composed of sedimentary rocks from the Mesozoic Hawasina ocean and the Samail ophiolite. In this window, the autochthonous sedimentary cover of the eastern Arabian Platform from the Late Neoproterozoic to the Cenomanian is well exposed. The oldest of these strata, the Hatat schists, were deformed into a NE-facing fold nappe during the upper Cretaceous. Within the overturned and thrusted lower limb of that fold nappe, we identified three small windows exposing stratigraphically younger Hiyam carbonates and Ordovician sandstone. The structural inventory of the windows and the surrounding area indicates three major tectonic phases. The first deformation led to NNE-SSW trending fold structures which probably formed simultaneously with the major fold nappe of the Hatat schists, followed by the extreme attenuation and thrusting of the lower limb of the fold nappe. The second phase was a gentle folding of the thrust with N-S oriented fold axes and third deformation phase that formed WNW-ESE oriented open folds. The windows are situated in the intersection of anticline axes of these two superposed fold generations and represent a mini basin-and-dome structure with an extension of just 1 km?×?1 km.