Search

Field relationships, mineralogy, petrology, geochemistry, geochronology, and Nd-Hf-O isotopes of the mafic-ultramafic rocks from the east part of the Qilian block are studied in the present work. The Aganzhen intrusive body only exposed in the Zhigoumen, Shiguanzi, Xianggoumen outcrops and includes Hornblende peridotite, wehrlite, olivine-bearing pyroxenite, hornblende-bearing pyroxenite, websterite, clinopyroxenite, hornblendite, olivine-bearing gabbro. The gabbroic rocks are also layered or massive cumulates with rock types varying continuously from noritic gabbro through hornblende gabbro to dioritic norite. Contact metamorphic zones are well developed between the Aganzhen intrusive body and the country rock. Major element contents of Aganzhen ultramafic-mafic rocks show subalkalic series and are characterized by low SiO2 contents (38.09-54.96 %), low TiO2 contents (0.09-0.72 %), low P2O5 contents (0.00-0.36 %) and alkali contents (Na2O+K2O 0.01-5.35 %), but high MgO contents (9.68-33.06 %), Ni contents (116-1505 ppm), Cr contents (713-2808 ppm). Similar LREE-rich pattern ((Ce/Yb)N =0.95-3.80 except two Samples) and tiny Eu anomaly (Eu/Eu* =0.6-1.2) indicate the Aganzhen ultramafic-mafic rocks have the same magma source. Trace elements are enriched in LILE (Rb, Th, U, K), relatively depleted in HFSE (Nb and Ta), and the La/Yb, Ce/Yb, Th/Yb, Nb/La, La/Sm values suggest the limited crustal contamination during the rise of the magma. The εNd (430 Ma) values are −6.9–+2.5 and TDM values are 3.6–1.4 Ga. The SHRIMP ages are 433±2 Ma for the Zhigoumen websterite(101-2101A), 434±3 Ma for Shiguanzi hornblendite(101-2104A) and 412±3 Ma for the Xianggoumen serpentinite(101-2107A). In situ zircon O-Hf isotope, the δ18O compositions of vary from +9.03 to +9.50 (except three points +11.33, +12.38, +12.44) and εHf(t) value is +0.29 to +4.13 for the Zhigoumen pyroxenite(101-2101A), the δ18O compositions of vary from +6.39 to +7.12 and εHf(t) value is +7.76 to +13.26 for Shiguanzi gabbro(101-2104A), and the δ18O compositions of vary from +4.68 to +5.31 and εHf(t) value of +0.28 to +2.79 for the Xianggoumen serpentinite(101-2107A). According to the above datum, we suggest that middle Paleozoic magmatisms last ~20 m.y. (434-412 Ma) on the northern margin of the Qilian Block was related to the Early Paleozoic continental collision between the Qilian and Alax blocks, and to subsequent subduction and thermal underplating.

The petrology, geochemistry, geochronology, and Sr–Nd–Hf isotopes of the backarc granitoids from the central part of the Qilian block are studied in the present work. Both S- and I-type granitoids are present. In petrographic classification, they are granite, alkali feldspar granite, felsic granite, diorite, quartz diorite, granodiorite, and albite syenite. The SHRIMP ages are 402–447 Ma for the S-type and 419–451 Ma for the I-type granitoids. They are mostly high-K calc-alkaline granitoids. The S-type granitoids are weakly to strongly peraluminous and are characterized by negative Eu anomalies (Eu/Eu* = 0.18–0.79). The I-type granitoids are metaluminous to weakly peraluminous and are characterized mostly by small negative to small positive Eu anomalies (Eu/Eu* = 0.71–1.16). The initial (87Sr/86Sr) values are 0.708848–0.713651 for the S-type and 0.704230–0.718108 for the I-type granitoids. The eNd(450 Ma) values are − 8.9–−4.1 and − 9.7–+ 1.9 for the S-type and I-type granitoids, respectively. The TDM values are 1.5–2.4 Ga for the S-type and 1.0–2.3 Ga for the I-type granitoids. For the Qilian block, the backarc granitoid magmatism took place approximately 60 million years after the onset of the southward subduction of the north Qilian oceanic lithosphere and lasted approximately 50 million years. Partial melting of the source rocks consisting of the Neoproterozoic metasedimentary rocks of the Huangyuan Group and the intruding lower Paleozoic basaltic rocks could produce the S-type granitoid magmas. Partial melting of basaltic rocks mixed with lower continental crustal materials could produce the I-type granitoid magmas. Major crustal growth occurred in the late Archean and Meso-Paleoproterozoic time for the Qilian block. The magma generation was primarily remelting of the crustal rocks with only little addition of the mantle materials after 1.0 Ga for the Qilian block.

The field occurrence, petrography, geochemistry, Sm–Nd isotopes, and geochronology of the Neoproterozoic granitoids exposed at Tuole, Huangyuan, Gahai, Haiyan, Riyueshan, Baokuhe, and Maxianshan in the Qilian block were studied. The Neoproterozoic granitoids in the Qilian block form two age groups: ca. 800 Ma and ca. 900 Ma. The ca. 800 Ma group granitoids consist of granodiorite and granite, whereas the ca. 900 Ma group granitoids quartz diorite, granodiorite, and potassic granite. They have intruded the Huangyuan Group in the basement sequence of the Qilian block. The foliations are moderately to well-developed and are concordant with those of the country rocks. The granitoids of both age groups all show enrichment in LREEs, but this enrichment is more pronounced in the ca. 800 Ma group than in the ca. 900 Ma group. All granitoids show negative europium anomalies, but the ca. 800 Ma group granitoids are more varied in Eu/Eu* values (0.23–0.83) than most of the ca. 900 Ma group granitoids (0.54–0.98). Spider diagrams of both groups also exhibit enrichment in large ion lithophile elements, i.e., Rb, Th, U, and K, and negative anomalies in Nb–Ta, Sr, P, Ti, and, with exception of the Maxianshan quartz diorite, Ba. The ɛNd(1 Ga) and TDM are −6.7 to −12.7 and 2.2–3.0 Ga for the ca. 800 Ma granitoids and are −4.3 to −5.2 and 2.0–2.3 Ga for the ca. 900 Ma granitoids. The granitoids of the ca. 900 Ma group are interpreted to have formed in an arc tectonomagmatic environment on an active continental margin, whereas those of the ca. 800 Ma group are thought to form in a continental rift environment. The ca. 900 Ma granitoids are I-type and could be formed from partial melting of K-rich metabasalt or eclogite with significant amount of mantle components at a pressure of 1–4 GPa in the lower crust. The strongly peraluminous or peraluminous Gahai granites and Haiyan granodiorite are S-type and could be derived from melting of clay-poor, mature psammitic sources. Similarities in age and tectonic environment of magma genesis suggest a correlation of Neoproterozoic granitoid magmatism between the Qilian and South China block. This correlation gives a strong support to the consideration that the two blocks might be unified in the Neoproterozoic. In other words, the Qilian block might be a part of the South China block or, more precisely, the Sibao orogen, in the Neoproterozoic time.

The amphibolite-facies metamorphosed mafic rocks exposed in the Maxianshan area, Qilian block, NW China, intruded granitic gneisses of the Maxianshan Group. In the field, they are seen to inject into and capture fragments of the surrounding granitic gneisses. Their rock types range from amphibole-rich amphibolite to plagioclase-rich amphibolite. Their whole-rock major element compositions suggest that their protoliths were subalkalic gabbroic rocks. The rare earth element patterns show slight light rare earth element enrichment with a very small europium negative anomaly. The amphibolites exhibit enrichment in large ion lithophile elements and negative anomalies of titanium, strontium, niobium and tantalum on the basis of their spider diagrams. The SHRIMP U–Pb ages of the zircon grains are 919 ± 10 Ma for the amphibole-rich amphibolite and 905 ± 6 Ma for the plagioclase-rich amphibolite, indicating the ages of protolith formation. The e Nd (910 Ma) values are in the range of −1.6 and −2.4, consistent with generation in a continental arc. Tectonic discriminations by whole-rock trace element compositions suggest that they originated in a continental arc environment. The gabbroic protoliths were metamorphosed under amphibolite-facies conditions. The Maxianshan amphibolite-facies metamorphosed mafic rocks could be correlated with the mafic–ultramafic rocks from marginal areas of the Yangtze block. This correlation implies that the Qilian block and marginal areas of the Yangtze block were active continental margins in the early Neoproterozoic and provides evidence for a strong tectonic affinity of the Qilian block with the South China block and even for a unified Qilian-South China block at that time. It is inferred that the Qilian block carrying Maxianshan mafic rocks underwent amphibolite-facies metamorphism after it was rifted away from the South China block in the late Neoproterozoic as a result of the breakup of the Rodinia supercontinent and collided with the Alaxa block in the early Paleozoic.

Adakitic intrusive rocks of ∼ 430–450 Ma were discovered in the North Qilian orogenic belt, the western section of the Central Orogenic System (COS) in China. These adakitic rocks were lower crust melts rather than slab melts as indicated by their crustal Ce/Pb, Nb/U, Ti/Eu, and Nd/Sm ratios and radiogenically enriched (87Sr/86Sr)i of 0.7053–0.7066 and eNd(t) of − 0.9 to − 1.7. While they are all characterized by low Yb ( 65) and (La/Yb)N (> 13.7) ratios, these adakitic rocks are classified into the low-MgO–Ni–Cr and high-MgO–Ni–Cr groups. The low-MgO samples were derived from partial melting of thickened lower crust, whereas the high-MgO samples were melts from delaminated lower crust, which subsequently interacted with mantle peridotite upon ascent. Adakitic rocks from the adjacent North Qinling orogenic belt also originated from thickened lower crust at ∼ 430 Ma. In addition, the North Qilian and North Qinling orogenic belts both consist of lithological assemblages varying from subduction-accretionary complexes at south to central arc assemblages, which include adakitic rocks, then to backarc phases at north. Such a sequence reflects northward subduction of the Qilian and Qinling oceans. In these two orogenic belts, the occurrence of adakitic rocks of common origin and ages together with the similarities in tectonic configurations and lithological assemblages are considered to be the evidence for the continuity between eastern Qilian and western Qinling, forming a > 1000 km Early Paleozoic orogenic belt. In such a tectonic configuration, the Qilian and Qinling oceans that subducted from south possibly represent parts of the large “Proto-Tethyan Ocean”. This inference is supported by the coexistence of Early Paleozoic coral and trilobite specimens from Asia, America and Australia in the North Qilian orogenic belt. Post-400 Ma volcanic rocks occur in the North Qinling orogenic belt but are absent in the North Qilian orogenic belt, indicating that these two orogenic belts underwent distinct evolution history after the closure of the Proto-Tethyan Ocean (∼ 420 Ma).

Field relationships, mineralogy and petrology, whole-rock chemistry, and age of the Zhamashi mafic–ultramafic intrusion in the North Qilian Mountains, northwest China, have been studied in the present work. The Zhamashi intrusive body consists of ultramafic, gabbroic, and dioritic rocks in a crudely concentrically zoned structure. The ultramafic rocks are layered cumulates with rock types varying continuously from dunite through wehrlite and olivine clinopyroxenite to clinopyroxenite. The gabbroic and dioritic rocks are also layered or massive cumulates with rock types varying continuously from noritic gabbro through hornblende gabbro to diorite. The ultramafic and adjoining gabbroic rocks are discontinuous in lithology and discordant in structure across the interface. The interface is steep, sharp, and fractured. Contact metamorphic zones are well developed between the Zhamashi intrusive body and the country rock. The concentrically zoned structure of the intrusive body and the intrusion into the continental crust are the two main pieces of evidence for considering that the Zhamashi intrusion is Alaskan-type. The mineral chemistry of the chromian spinels (Cr-spinels) and clinopyroxenes, and the variation trend of the whole-rock compositional plot in the (Na2O + K2O)–FeO–MgO (AFM) diagram are also supportive of this consideration. The age of the Zhamashi intrusive body, determined with sensitive high mass-resolution ion microprobe on the zircon grains, is 513.0 ± 4.5 Ma. Parental magma of the Zhamashi intrusion is compositionally close to the primitive magma produced by partial melting of the mantle peridotite. It was differentiated by fractional crystallization at low total pressure and under H2O-rich conditions in an arc environment to form all the major rock types. The concentrically zoned structure of the Zhamashi intrusive body was constructed in two stages: formation of a stratiform-type layered sequence, followed by diapiric re-emplacement. The occurrence of the Alaskan-type intrusion suggests an active continental margin and Cambrian arc magmatism for the northern margin of the Qilian Block.

Analytical electron microscopic observations have been carried out on a garnet peridotite from the Maobei area, Sulu ultrahigh-pressure terrane. The results showed that olivine in this garnet peridotite (5.3–6.6 GPa; 853–957 °C), contains precipitates of chromian magnetite and chromian-titanian hematite at dislocations and (001) faults. Specific crystallographic relationships were determined between these precipitates and the olivine host, viz. [101] Mt //[001] Ol , [110] Mt //[011] Ol , and [011] Mt //[011] Ol ; and [0001] Hm //[100] Ol and [1010] Hm //[001] Ol . These oriented oxides are not associated with silicate/silica phases and therefore cannot be accounted for by the mechanism of olivine oxidation. It is postulated that these magnetite and hematite precipitates most likely have resulted from dehydrogenation-oxidation of nominally anhydrous mantle olivine during rock exhumation. In view of the contrasting diffusion rates of H and Fe in the olivine lattice, it is suggested that the formation process might actually take place in steps. Hydrogen diffusion with concomitant quantitative oxidation of Fe 2+ to Fe 3+ in olivine occurred early during initial rock exhumation and was followed by slow Fe diffusion forming magnetite/hematite at stacking faults and dislocations within the olivine lattice. Two requirements are essential under such a scenario: an ample amount of H content of the olivine, and an appropriate exhumation rate, probably in the range of 6–11 mm/year, of the host rock. It is also noted that such dehydrogenation-oxidation processes may hamper a correct estimate of the actual P-T conditions and mantle oxidation state based on mineral chemistries present in mantle eclogite/peridotite. The present study demonstrates that oriented mineral inclusions may not necessarily form through exsolution processes sensu stricto, but may form through a series of more complicated reaction mechanisms.

Origin and tectonic evolution of the Qilian Precambrian basement on NW China were investigated using zircon U-Pb ages with collaborating stratigraphic and paleontological evidence. Zircon grains were separated from two schists, two granitic gneisses and one mylonized gneiss and dated with SHRIMP. Seventy percent of sixty-one detrital zircon ages from two schists ranges from 0.88 Ga to 3.09 Ga, mostly within 1.0 Ga to 1.8 Ga with a peak at 1.6 Ga to 1.8 Ga, and twenty percent varies from 2.0 Ga to 2.5 Ga. A few falls in the Archean and Neoproterozoic periods. The two granitic gneisses were dated 930±8 Ma and 918±14 Ma, whereas the mylonized granitic gneiss was dated 790±12 Ma. These ages represent two periods of magmatisms, which can be correlated with the early and late stages of magmatisms associated with the Jinningian movement on the Yangtze Blocks.

The Dongcaohe ophiolite, located at the south of the North Qilian subduction complexes, is a tectonic block with an exposed area of about 3 km×6 km. It consists of an intrusive section overlain by an extrusive section. The lower part of the intrusive section consists of cyclic layers of cumulate dunites, troctolites, anorthosites, anorthositic gabbros, and gabbros with small discordant dunite and troctolite bodies. This layered sequence grades upward to isotropic gabbros and gabbronorites, which are overlain by the extrusive sequence of diabasic sheeted dikes and basaltic lavas. The overall mineral crystallization sequence was olivine±Cr-spinel, plagioclase, clinopyroxene, orthopyroxene, and Fe-Ti oxides. The Cr-spinel (Mg#: 42–66, Cr#: 41–57) in these layered cumulates and present-day abyssal peridotites have similar compositions. Also, the compositional variations of the plagioclase and clinopyroxene in the intrusive section reflect crystallization from melts compositionally similar to the present-day ocean basalts. Moreover, the rare earth element (REE) and multi-element distribution patterns of the intrusive and extrusive lithologies in the Dongcaohe ophiolite are consistent with crystallization of mid-ocean ridge basalts. The zircon grains separated from the gabbronorite have an SHRIMP average 206Pb/238U weighted age of 497 ± 7 Ma, which is considered as the tectonic emplacement age of the Dongcaohe ophiolite. The field occurrence, mineral and whole-rock compositions indicate that the Dongcaohe ophiolite represents a well-persevered oceanic crustal fragment composed of a complete oceanic crustal section of layered cumulates at bottom upgrading through isotropic cumulates to sheeted dikes and lava flows.

Diopside, enstatite, pargasite, aspidolite, analcime, chlorite, serpentine, titanite, andradite, and grossular are found as mineral inclusions in grains of chromian spinel in the troctolites and dunites of the layered cumulate series and discordant layered bodies, Dongcaohe ophiolite, in the North Qilian Mountains, Qinghai Province, northwestern China. The mineral inclusions in the layered cumulate series are remarkably different in species and composition from those in the discordant layered bodies. Diopside, the liquidus phase, was enclosed by the growing crystals of chromian spinel. Enstatite, pargasite and aspidolite were products of the closed-system reaction between the trapped mineral phases and trapped residual melt. Analcime crystallized or precipitated directly from the evolved entrapped melt or hydrothermal solution in the inclusions. Chlorite, serpentine, titanite, andradite, and grossular are of secondary or metamorphic origin and were produced by alteration of olivine, pyroxene, or amphibole during postmagmatic hydrothermal alteration or subsequent seafloor metamorphism of the ophiolite. Syntexis of the lower oceanic crust was involved in the formation of a modified magma, which solidified as the discordant layered bodies scattered within the layered cumulate series of the Dongcaohe ophiolite.

An early Paleozoic ultrahigh pressure metamorphic belt occurs in the north Qaidam–Altun mountains and was offset about 400 km southwestward by the Altyn Tagh strike-slip fault. Eclogites in the belt consist of major basaltic and minor picritic rock types and can be subdivided into three groups: high TiO2 (2–5 wt%), medium TiO2 (1–2 wt%) and low TiO2 (

Serpentinites are a common component of ophiolitic assemblages in the Qilian fold belt. Samples associated with MORB-like basalts in the northern low-grade blueschist belt of the North Qilian terrane have lherzolitic and pyroxenitic compositions, high eNd(T) values of +5.5 to +6.9, and likely represent residues from melting along spreading ridge segments. Despite evidence for incompatible element enrichment by hydrothermal fluids which had interacted with the ocean floor sediment pile, samples at the Ayougou locality retain original isotope Sm–Nd systematics and yield a Silurian age of 422±20 Ma with eNd(T)=+6.8. Serpentinites in the southern high-grade blueschist belt of the North Qilian terrane and from the Lha-Jyi terrane of the South/Central Qilian belt are associated with volcanic arc rocks, and include lherzolitic, harzburgitic and dunitic compositions characterised by low eNd(T) values of +3.8 to −2.2. Harzburgitic samples from these localities display U-shaped REE profiles, and weakly developed negative Nb-anomalies superimposed on incompatible element enrichments, and are interpreted as remnants of oceanic lithosphere metasomatised by slab-derived fluids in a supra-subduction zone environment. Anomalous enrichments in the mobile elements U, Sr and Ba in serpentinites throughout the Qilian belt may be attributed to post-obduction alteration by groundwater.

SHRIMP U-Pb dating was carried out on zircons from the Niuxinshan gneissic granite and the Leigongshan gneissic tonalite in the North Qilian orogen, NW China. The results yield weighted averaged 206Pb/238U ages of 776±10 Ma and 774±23 Ma respectively. Igneous morphology, oscillatory zonings, and relatively high Th/U ratios for these zircons suggest magmatic origin. These ages are interpreted as timing of magma emplacement and thus representing a Neoproterozoic (∼775 Ma) magmatic event in the North Qilian area. It is suggested that this magmatism is probably related to breakup of the supercontinent Rodinia. This finding, together with the similar ages of 750 to 800 Ma reported for neighboring terranes of South-central Qilian and North Qaidam, is of significance to understanding of the Rodinia evolution in West China.

The corrosion resistance and microstructure of four highalumina and two chromia-containing high-alumina refractories were evaluated. Analysis of the refractories after exposure in the rotary slag test showed that the chromia-containing products had better slag resistance than the chromia-free products. Three factors contributed to this conclusion, including formation of a Cr-spinel layer at the slag/refractory interface, formation of an impermeable mullite layer just below the interface, and improved matrix bonding from chromia-alumina solid-solution formation.

The stability of ilmenite and titanomagnetite in the presence of CO2 gas was evaluated thermodynamically. Ilmenite with a composition Ilm90Hm10 breaks down to the assemblage siderite-hematite-anatase below 270° C at 1,000 bars and below 300° C at 2,000 bars. Titanomagnetite with a composition Usp50Mt50 breaks down to the same assemblage below 345° C at 1,000 bars and below 385° C at 2,000 bars. During the low-grade metamorphism of basaltic rocks, a CO2 partial pressure of only a few hundreds bars is sufficient to alter ilmenite and titanomagnetite to the assemblage siderite-hematite-anatase.

Desert varnish samples from Deep Springs Valley and Death Valley in California were studied with the use of the electron microprobe. Varnish can be interpreted as having two layers, an inner subordinate layer rich in $SiO_{2}$ and usually $Al_{2}O_{3}$ and an outer main layer rich in FeO and MnO. The subordinate layer may be altered rock which has lost all evidence of crystal structure and to which Fe and Mn have been added. These layers are not distinguishable optically. In varnish on quartzites, FeO, MnO, $Al_{2}O_{3}$, and $K_{2}O$ all increase in concentration outward through the varnish from the rock-varnish contact. However, in varnish on argillites, $Al_{2}O_{3}, K_{2}O$, and sometimes FeO decrease in concentration outward. Comparison of these variations with data on the bulk composition of the fresh rock suggests that elements which increase outward may be supplied largely from external sources and that elements which decrease outward may be supplied predominantly from the underlying rock. The FeO...

On propose des mecanismes expliquant pourquoi la poudre d'aluminium previent la formation de trichites de SiC a partir de Si metallique et de brai de houille lors du frittage des refractaires