2019 Vol. 30, No. 3
To discuss the nature of the compositional heterogeneity of the peridotite massifs of the Polar Urals (Russia), the geochemical study by LA-ICP-MS of pyroxenes and amphiboles from these mantle formations was performed. The trace element compositions in clinopyroxenes indicate the existence of the mantle protolith of two types. The first protolith type, represented by lherzolites and diopside harzburgites, was originated from the partial melting (5%-10%) under the spinel facies conditions, while the second one, represented by diopside harzburgites, was formed under the polybaric partial melting (17%-19%) under garnet and spinel facies conditions. Subsequently, the mantle peridotite protolith was subject to fluidinduced partial melting in the suprasubduction setting that was resulted in the formation of harzburgites. Being affected by penetrating melts and fluids peridotites experienced the refertilization (LREE enrichment of clinopyroxenes) and high-temperature hydratation with subsequent development of pargasite and Mg amphibole. The high-T fluid-induced metamorphism at the subduction zone was accompanied by the formation of metaperidotites with clinochlore and REE-depleted tremolite.
On the Moon and Mars olivine of probable mantle origin is detected at rims of large Late Heavy Bombardment (LHB) age impact basins for which excavation depth estimates exceed crustal thickness estimates. But lunar Crisium size impact basins are not recognized on Earth nor expected in the Phanerozoic from conventional interpretations of crater size frequency distributions. In this study several large circular to elliptical basin structures on Earth, for which hypothesized impact excavation depth would greatly exceed crustal thickness, are examined for the presence of exposed lithospheric mantle, expressed as ophiolite, at the rims. Three Phanerozoic impact basins, modified by plate tectonics and tentatively correlated with "ophiolite obduction" plus global extinction events, are proposed here. These tentatively suggested Phanerozoic impact basins are:(1) Yucatan Basin/Puerto Rico Trench with a Greater Antilles ophiolite rim. Cretaceous-Paleogene Boundary global extinction may correlate with Maastrichtian ophiolite obduction in Southeast Cuba. (2) Loyalty Basin with a New Caledonia ophiolite plus d'Entrecasteaux Ridge rim. Late Eocene global extinction may correlate with obduction of the New Caledonia Peridotite Nappe. (3) Sulu Sea Basin with a Palawan, Sabah etc. ophiolite rim. The Middle Miocene Disruption Event may correlate with ophiolite obduction plus ophiolitic mélange emplacement in Sabah and in Palawan. These originally circular to elliptical belts of exposed lithospheric mantle may serve as strain markers for relative plate motions in the vicinity of plate boundaries during post-impact geologic times. It is further speculated that plate boundaries may be initiated and/or modified by such impacts.
Ophiolites play a key role in understanding subduction-accretion-collision processes. Herein, we discuss origin and metamorphic evolution of an enigmatic, Neoproterozoic ophiolite candidate-the mafic-ultramafic Songshugou Complex, Qinling belt, China-summarizing published thermobarometry, U/Pb geochronology, and geochemistry and presenting new phase equilibrium modeling. Garnet, rarely preserved in amphibolites of the Songshugou Complex, has prograde zoning and low-pyrope cores[Alm54-71 (Grs+And)25-30Prp1-6Sps5-12]. It formed at quartz eclogite facies conditions of 1.93-2.54 GPa, 462-542¦. During exhumation, garnet first was mantled by plagioclase-rich coronas at about 0.7-1.2 GPa, 660-710¦. During an isothermal uplift to 0.5-0.8 GPa, these coronas evolved widely into σ-shaped aggregates and eventually into whitish ribbons oriented with a steeply southwest dipping mineral stretching lineation. The exhumation into middle-upper crustal levels proceeded till the Late Devonian. The oceanic protoliths of the eclogites were emplaced into continental crust in the Neoproterozoic and dragged into a subduction zone in North Qinling in the Cambrian. The ultramafic rocks of the Songshugou Complex were not subducted with the mafic rocks in a coherent block given the absence of garnet but ubiquitous occurrence of spinel implies a P maximum of~1.7 GPa. Rather, mafic and ultramafic rocks belonged to downgoing and overriding plate, respectively. They were juxtaposed at 0.8-1.7 GPa at Early Ordovician time.
The Songshugou peridotite massif is located in the north of Shangdan suture zone, North Qinling orogenic belt of Central China. The massif is mainly composed of fine-grained mylonitic dunites, coarse-grained dunites, fine-and coarse-grained harzburgites, and minor clinopyroxenites. The coarsegrained dunites as well as parts of the harzburgites host small-scale chromitites. Chromite grains from various textural types of chromitites and dunites pervasively contain primary and secondary silicate inclusions. Primary inclusions are dominated by monophase olivine, with minor clinopyroxene and a few multiphase mineral assemblages consisting of olivine and clinopyroxene. Secondary inclusions, mainly Cr-chlorite and tremolite, show irregular crystal shapes. Besides, Cr2O3 contents (0.08 wt.%-0.71 wt.%) of primary olivine inclusions are remarkably higher than those of interstitial olivine (< 0.1 wt.%). Chromites in the Songshugou peridotite massif are high-Cr type, with Cr# and Mg# values ranging of 67.5-87.6, and 23.4-41.2, respectively. The Cr-chlorite, formed by reactions between olivine and chromite in the presence of fluid under middle temperature, indicates the Songshugou peridotite massif has undergone alteration/metamorphism process during emplacement. Chromite grains are modified by these processes, resulting in the various degrees of enrichment of Fe2O3, Cr2O3, Zn, Co and Mn, depletion of MgO, Al2O3, Ga, Ti and Ni. Due to low silicate/chromite ratios in the massive ores, chromites from them are slightly influenced by alteration/metamorphism and thus preserve the pristine magmatic compositions. The parental magma calculated based on them has 11.17 wt.%-13.57 wt.% Al2O3 and 0.15 wt.%-0.27 wt.% TiO2, which is similar to the parental melts of high-Cr chromitites from elsewhere and comparable with those of boninites. Combined with informations from previous studies, major and trace elements geochemistry of chromite, as well as the nature of the parental magma, it can be revealed that the Songshugou chromitities formed in a supra-subduction zone environment.
The Qinling Complex from the Qinling orogenic belt was generally considered to be part of the Caledonian orogeny, however information of the Grenville event of the Qinling Complex has been poorly recognized. Two granite samples of greenschist-facies and two paragneiss samples of amphibolite- faices are identified from the Qinling Complex. The granites occur along the regional gneissosity of the Qinling Complex, thus it is suggested that their magmatic zircon ages (~970 Ma) mark the lower boundary of the main metamorphic age (amphibolite-granulite facies). In addition, some early-formed metamorphic zircons (~1 000 Ma) are distinguished in the granites, which may reflect the information about the source area of the granites. Therefore the major metamorphism of amphibolite-granulite facies in the Qinling Complex is constrained at Early Neoproterozoic (~1 000 Ma), not Early Paleozoic as conventionally considered. In the Early Paleozoic, the Qinling Complex was characterized by multiple extension- shear activities, overprint of greenschist-facies metamorphism and emplacement of extensive granites. These granites and related thermal events could reset the U-Pb isotopic system of the early-formed zircons, leading to the apparent zircon ages younger than their protolith age. As a result, the Qinling Complex is a modified Early Neoproterozoic orognenic belt or an independent block, which became a continental margin arc during the Early Paleozoic, being accompanied by metamorphism, deformation, and emplacement of continental arc granites. The Erlangping, Kuanping, and Taowan groups to the north of the Qinling Complex show more intensive deformation of the Caledonian, and their oblique subduction towards the Qinling Complex caused the formation of eclogites. Afterwards, the Qinling Complex was amalgamated with the Erlangping, Kuanping, and Taowan groups, which all experienced the same Caledonian orogeny, and possible later orogenies.
The Sumdo eclogite-bearing (U)HP metamorphic belt extends over 100 km across the middle part of the Lhasa terrane in southern Tibet, which forms a Permian-Triassic oceanic subduction zone between the south and the north Lhasa sub-terranes, leading to the reinterpretation of the tectonic evolution of the Lhasa terrane in the Tibetan-Himalayan orogeny. Previous studies show that there are significant differences in temperature and pressure conditions of the eclogites in four areas, e.g., Sumdo, Xindaduo, Bailang and Jilang areas. Studying the peak metamorphic P-T conditions and path of eclogite in the Sumdo belt is of great significance to reveal the subduction and exhumation mechanism of Paleo-Tethys Ocean in the Lhasa terrane. In this contribution, eclogite in the Jilang area of the Sumdo belt is chosen as an example to study its metamorphic evolution. The mineral assemblage of the eclogite is garnet, omphacite, phengite, hornblende, epidote, quartz and minor biotite. Garnet has a "dirty" core with abundant inclusions such as epidote, amphibole, plagioclase and a "clear" rim with few inclusions of omphacite and phengite. From the core to the rim, pyrope content in garnet increases while grossular content decreases, showing typical growth zoning. The rim of garnet is wrapped by the pargasite+plagioclase corona, showing amphibolite facies overprint during retrogression. Three stages of metamorphism are inferred as (1) prograde stage, represented by the core of garnet and mineral inclusions therein; (2) peak stage, represented by the garnet rim, omphacite, lawsonite, phengite, and quartz; (3) retrograde stage characterized by decomposition of lawsonite to zoisite, followed by symplectite of omphacite and corona rimmed garnet. A P-T pseudosection contoured with isopleths of grossular and pyrope contents in garnet is used to constrain the near peak P-T condition at 2.85 GPa, 575 C. In general, the Jilang eclogite shows a clockwise P-T path with a near isothermal decompression process during exhumation. Combined with the age peaks of 583, 911, and 1 134 Ma from the detrital zircons of the country metaquartzite, a continental margin material involving exhumation process at shallow depth after the subduction channel exhumation is inferred for the Jilang eclogite and may further indicate that the subduction direction of the Sumdo eclogite belt is from north to south.
The Himalayan leucogranites provide insights into the partial melting behavior of relatively deeper crustal rocks and tectono-magmatic history of the Himalayan Orogen. The Paiku leucogranites of northern Himalaya can be subdivided into two-mica leucogranite (TML), garnet-bearing leucogranite (GL), cordierite-bearing leucogranite (CL), and tourmaline-bearing leucogranite (TL). All of them are high-K, peraluminous, calc-alkalic to alkali-calcic rocks. They are enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE), and show pronounced negative anomalies of Sr, Ba, K and Ti, but positive anomalies of Nb and Rb. LA-ICP-MS U-Pb zircon dating of one TML, one GL, and two CL samples yielded variable 206Pb/238U ages ranging from 23.6 to 16.1 Ma, indicating the Paiku leucogranites underwent a low degree of partial melting process. Combining with previous studies, we suggest the Paiku leucogranites were derived from partial melting of metasedimentary rocks of the Higher Himalayan Sequence (HHS). The GL and TL mainly resulted from the muscovite-dehydration melting, whereas the TML and CL were mainly derived from the biotite-dehydration melting. Finally, it is concluded that the Paiku leucogranites were probably formed during the subduction of the Indian crust.
The Yardoi dome is located in the eastern end of the northwest-southeast extending North Himalayan domes (NHD). The dome exposes a granite pluton in the core and three lithologictectonic units separated by the upper detachment fault and the lower detachment fault. The Yardoi detachment fault (YDF), corresponding to the lower detachment fault, is a 800 m strongly deformed top-NW shear zone. LA-ICP-MS zircon U-Pb dating yielded a crystallization ages of 19.57±0.23 to 15.5±0.11 Ma for the leucogranite dyke swarm, which indicates that the ductile motion along the YDF began at ca. 20 Ma. The 40Ar/39Ar muscovite ages of 14.05±0.2 to 13.2±0.2 Ma and the 40Ar/39Ar biotite age of 13.15±0.2 Ma, suggest that the exhumation led to cooling through the 370℃ Ar closure temperature in muscovite at~14 Ma to the 335℃ Ar closure temperature in biotite at~13 Ma. Our new geochronological data from the Yardoi dome and other domes in the Tethyan Himalayan Sequences suggest that the ductile deformation in the region began at or before~36 Ma in a deep tectonic level, resulting in southward ductile flow at the mid-crustal tectonic level that continued from 20 to 13 Ma. Comparing the Yardoi dome to other domes in the NHD, the cooling ages show a clear diachronism and they are progressively younger from the West Himalayan to the East Himalayan.
The clinopyroxene amphibolite from the Bailang terrane is located in the central section of the Yarlung Zangbo suture zone (YZSZ), southern Tibet. The study of it is expected to provide important clues for the subduction of the Neo-Tethyan Ocean below the Asian Plate and thus for better understanding of the development of the India-Asia collision zone. Based on integrated textural, mineral compositional, metamorphic reaction history and geothermobarometric studies of the clinopyroxene amphibolite within a serpentinite mélange, four overprinted metamorphic stages are established. They are the first metamorphic record of M1 stage indicated by a relict assemblage of plagioclase+clinopyroxene+amphibole, an early M2 stage characterized by an assemblage of medium-grained clinopyroxene+amphibole+plagioclase+quartz as well as rutile inclusion in titanite, which is formed during burial process, an isobaric cooling M3 stage which is characterized by an assemblage of clinopyroxene+amphibole+plagioclase+titanite, and a decomposing retrograde stage M4, which is represented by the amphibolite+plagioclase symplectite+titanite+ rutile+quartz. By applying the THERMOCALC (versions 6.2 and 6.3) technique in the NCFMASHTO system, the P-T conditions estimated from M1 to M4 stages are ca. 8.6 kbar/880℃, 10.8-13.4 kbar/800-840℃, 12.7-13.2 kbar/650-660℃ and < 11.2 kbar/640℃, respectively. The mineral assemblages and their P-T conditions define a counterclockwise P-T path for the clinopyroxene amphibolite of the Xigaze ophiolite, suggesting that the rocks underwent a cooling process during burial from magmatic protolith, and a decompressing stage after the pressure peak metamorphic conditions, which implies that the Bailang terrane of the Xigaze ophiolite may have experienced subduction/collision-related tectonic processes. The peak metamorphism reaches to the transitional P-T conditions among amphibolite facies, granulite facies and eclogite facies with a burial depth of 30-40 km. After exhumation of the ophiolitic unit to the shallow crustal levels, the clinopyroxene amphibolite exposes to a high fO2 condition on the basis of the stable epidotebearing assemblage in the T-MO2 diagrams. A late subgreenschist facies overprinting subsequently occurs, the relevant mineral assemblage is prehnite+albite+chlorite+epidote+quartz.
Helium isotopic compositions are considered to be ideal tracers to identify whether mantle materials have been added to crustal rocks or fluids. In this paper, we present the helium isotopic compositions of the Songduo eclogites in the Lhasa terrane, Tibet. We found that garnet and omphacite in the eclogites have different helium retention characteristics. The 4He content of most omphacite grains are about 10-20 times of that of garnet, suggesting that omphacite has a higher ability to capture 4He than garnet. Similarly, there is about 10-20 times difference in 3He content between omphacite and garnet in the same eclogite samples. The 3He/4He ratios of garnet and omphacite in these rocks range from 0.27 to 0.60 Ra (relative to the modern air 3He/4He ratio, 1.4×10-6). These ratios are within the range of both mantle-and crust-derived helium, suggesting mixed sources. The Songduo eclogites have much higher 3 He/4He ratios than those observed in the Dabie eclogites of eastern China. Such high ratios are typically thought to be associated with deep mantle sources. We cautiously conclude that deep mantle materials might have been involved during the formation of the Songduo eclogites.
Post-collisional potassic and ultrapotassic volcanic rocks are widely distributed across the Tibetan Plateau, and they are considered to be indicators of evolving mantle dynamics. A suite of potassic basalts younger than 55 Ma from the Saga area of western Tibet has been reported. The geochemical characteristics of these rocks distinguish themselves from other potassic-ultrapotassic volcanic rocks in Tibet, such as positive Nb, Ta, and Ti anomalies and strong enrichment in large ion lithophile elements (LILE), suggesting that phlogopite, rutile and/or sphene might have originated from the mantle source. These basalts are also characterized by a very wide range of 87Sr/86Sr ratios (0.709 043-0.711 915) and relatively high 143Nd/144Nd ratios (0.512 426-0.512 470, εNd=-4.60 to -3.87). We propose a petrogenetic model for the Saga potassic rocks in which the lithospheric mantle source was infiltrated by a volatilerich (H2O, CO2) and low-degree silicate melt derived from the asthenosphere in the Middle to Late Proterozoic. After the initial Indo-Asian collision, Neo-Tethyan slab breakoff resulted in the partial melting of the previously metasomatized lithospheric mantle and the formation of the Saga potassic rocks. It is likely that the eruption of these volcanic rocks lasted at least 10 Ma.
The granulitized eclogites from the Luliangshan terrane of the North Qaidam UHP metamorphic belt occur as lenses within pelitic gneisses and orthogneisses. Combined petrologic data and phase equilibrium modeling indicate a multi-stage metamorphic history of the granulitized eclogites:(1) an earlier eclogite facies metamorphism (P>18.5 kbar, T> 830℃) is deduced from omphacite relics in the matrix and rare omphacite inclusions within garnet. The possible assemblage is garnet+omphacite+rutile+ quartz; (2) the early stage of high pressure granulite facies assemblages (garnet+clinopyroxene+ plagioclase+rutile+quartz+liquid) developed in the early decompression process has a P-T regime of 17.5 kbar and 852-858℃, constrained by plagioclase and clinopyroxene inclusions in garnet. The late stage of high pressure granulite assemblages (garnet+clinopyroxene+amphibole+plagioclase+rutile+quartz+liquid) records an isothermal decompression process with the pressure successively declining from 17.5 to 14.7 kbar and to 11.3 kbar at 858℃; (3) the later medium pressure granulite facies assemblage (garnet+ orthopyroxene+clinopyroxene+amphibole+plagioclase+ilmenite+liquid+quartz) indicates a drop in pressure and rise in temperature at P-T conditions of 7.6-7.7 kbar and 878-883℃; (4) retrogressive amphibolite facies stage, which is represented by amphibole+plagioclase kelyphitic rims around garnet, formed under conditions of < 5 kbar and < 650℃. The preservation of medium pressure granulite facies assemblage and the garnet composition feature constrain a following isobaric cooling path during late exhumation. This process suggests a clockwise P-T path and indicates that the granulitized eclogites record a high grade "Barrovian" metamorphic overprint at the middle-lower crust during exhumation. The present data show that the Luliangshan terrane is a "hot" HP-UHP terrane.
The kyanite-bearing garnet pelitic gneiss from the Jianggalesayi area in southern Altyn Tagh high pressure/ultra-high pressure belt was proved to have been experienced UHP metamorphism (>12 GPa) by the discovery of kyanite and spinel exsolution microstructure in quartz (precursor stishovite). In this study, three stages of retrograded metamorphism (M2-M4) after the UHP metamorphism (M1) were identified for the UHP pelitic gneiss. The HP granulite-facies stage (M2) was characterized by the mineral assemblage of garnet+kyanite+K-feldspar+rutile+quartz±ilmenite, recording the P-T condition of >1.12 GPa and~850-930℃. The granulite-facies stage (M3) was represented by the mineral assemblage of garnet rim+K-feldspar+sillimanite (Sill1)+biotite (Bt1)+plagioclase (Pl1)+ilmenite+quartz, and confined under P-T conditions of 0.5-0.8 GPa and~770-795℃. The late cooling stage M4 was accompanied by the appearance of fine-grained Pl2, Sill2 and Bt2 in the matrix, and the P-T conditions were 0.4-0.6 GPa and < 675℃. A clockwised P-T path was obtained for the pelitic gneiss in the P-T pseudosection, which showed a deep subduction/collision processes with subsequent exhumation and cooling. Combined with the corresponding multistage metamorphic assemblages, the age dating results implied that the zircons from the gneiss have integrated the recording peak metamorphic (M1, 484±3 Ma) and retrograded metamorphic ages (M2 to M3, 450±2 Ma). There was about 32 Ma interval during the first exhumation from the upper mantle depth (>350 km) to the lower crust depth (~40-20 km), resulting in an average exhumation rate of 9.11-9.70 mm/yr. In the southern Altyn Tagh region, the HP and UHP rocks from different areas had identical peak metamorphic ages. Therefore, contemporary UHP and HP rocks with different metamorphic evolutions were recognized coexisting in the same orogenic belt, which can be interpreted by the model of subduction channel. The continental crustal were subducted to different depths along the direction of the subduction channels at~500 Ma, suffered different grade metamorphism, and then returned to the surface along the subduction channel.
To reveal the petrological characteristics, metamorphic evolution history and tectonic setting of the pelitic granulites from Ailaoshan Orogen, West Yunnan, China, a comprehensive study in mineral chemistry, petrogeochemistry and geochronology studies is presented in this paper. Two metamorphic stages of the granulites can be established:(1) the peak metamorphism recorded by the mineral assemblage of garnet, kyanite, K-feldspar and rutile, and the initial retrograde metamorphism shown by the mineral assemblage of garnet, sillimanite, sapphirine, spinel, K-feldspar, plagioclase and biotite; (2) the superimposed metamorphism recorded by the mineral assemblage of biotite, muscovite, plagioclase, quartz and ilmenite. Zircon LA-ICP-MS U-Pb dating indicates that the protolith of the granulite was deposited after 337 Ma. The initial retrograde metamorphism occurred at P-T conditions of 8.6-12 kbar at 850-920℃ estimated by mineral assemblages, the low pressure limit of kyanite stability and GBPQ geothermobarometer in Indosinian (about 235 Ma), and the late superimposed metamorphism occurred at the P-T condition of 3.5-3.9 kbar at 572-576℃ estimated by GBPQ geothermobarometer since 33 Ma. The first stage was related to the amalgamation of the South China and Indochina blocks during the Triassic, and the second stage was possibly related with the large scale sinistral slip-shearing since the Oligocene. It is inferred that the upper continental crust was subducted/underthrusted to the lower continental crust (deeper than 30 km) and underwent granulite-facies metamorphism and then quickly exhumed to the middle-upper crust (10-12 km) and initial retrograde metamorphism occurred due to the collision of the Indochina and South China blocks during Indosinian, which was followed by superimposition of the second stage of metamorphism since the Oligocene.
Syn-collapse magmatism is a critical issue for evolution of the continental orogen. The Dabie Orogen is a typical orogen which was suffered from a complete collapse. Two kinds of granitoids, namely, the coarse-grained diorite and the fine-grained granite, are recognized at the center of the Luotian extensional dome, providing an opportunity to decipher the syn-collapse magmatism in the Dabie Orogen. The diorites (125±3 Ma) are high K calc-alkaline rocks, with low SiO2 (51.9 wt.%-56.6 wt.%) and high MgO (3.5 wt.%-4.0 wt.%) contents. They are enriched in LREE and LILEs (e.g., Ba, K, Rb) and depleted in HFSEs (e.g., Ta, Nb, and Hf) with low ratio of Sr/Y (30.82-46.89). The granites (118±2 Ma) are shoshonite series rocks, with relatively high SiO2 (68.9 wt.%-72.6 wt.%) and low MgO (0.32 wt.%-0.66 wt.%) contents. They are also enriched in LREE and LILEs with weakly negative Eu anomalies (δEu=0.81-0.85), and are depleted in HFSEs with low Sr contents (338 ppm-477 ppm) and Sr/Y ratios (23.80-33.13). Therefore, the two kinds of granitoids have no geochemical characteristics of adakitic rocks, suggesting that they were generated from a normal or thinned crust level. The diorites have quite negative zircon εHf(t) values (-18.4 to -21.1), suggesting they were from partial melting of the mafic lower continental crust. The granites have relatively higher zircon εHf(t) values (-14.4 to -18.1). The granites also contains a series of old inherited zircon cores, such as two upper intercept ages of 2 628±41 and 1 840±37 Ma, and a concordant age of 807±9 Ma. All these features suggest that the granites were generated from partial melting of the felsic middle-lower continental crust. Thus, the Huilanshan Early Cretaceous granitoids coupled with the Luotian extensional dome revealed the collapsed process of the Dabie Orogen.
Archean high-K granitoids, generally formed after tonalite-trondhjemite-granodiorite (TTGs), are important for understanding crustal reworking of ancient cratons. The Linshan Archean high-K granitoids from the southern Trans-North China Orogen (TNCO) provide a window into the continental crustal evolution of the North China Craton (NCC). They mainly consist of monzogranite and granodiorite which were formed during 2 542-2 503 Ma. The high-K granitoids have high SiO2 (65.86 wt.%-78.08 wt.%), K2O (3.29 wt.%-7.62 wt.%) and low P2O5 (0.01 wt.%-0.27 wt.%). They display right inclined REE patterns with negative Eu anomalies (Eu/Eu*=0.20-0.81). Their spider diagram is characterized by enrichment of Rb, K, Th, U and depletion of Nb, Ta, Zr, Ti. The rocks have positive and variable zircon εHf(t) (+2.5 to +6.6) and whole-rock εNd(t) (+0.7 to +4.5) with two-stage model ages (TDM2Hf=2.87-2.64 Ga; TDM2Nd=2.77-2.50 Ga) similar to those of the Archean TTG-type rocks, amphibolites and diorites in the area. These evidences suggest that the high-K granitoids were produced by partial melting of juvenile crustal rocks. The Linshan high-K granitoids show relatively high whole-rock zircon saturation temperatures (694-889℃) and low Sr/Y ratios (0.27-21.1), indicating low pressure partial melting. Combined with other geological evidences, the Linshan high-K granitoids are suggested to have been produced by partial melting of the continental crust in a post-collision extensional environment after an arc-continent collision. Thus, the NCC did not amalgamate together until ca. 2.5 Ga. Compiled zircon U-Pb ages and Hf isotopes reveal that the ca. 2.5 Ga magmatism represents reworking of the continental crust.
The orogenic peridotites can be subdivided into crust-and mantle-derived types. They record complex geological processes in subduction and collision zones. The crust-derived peridotites are derived from cumulates crystallized from ultramafic-mafic magmas in deep continental crust, an early mantle-crust interaction, prior to subduction. The mantle-derived orogenic peridotites are originated from subcontinental lithospheric mantle (SCLM) wedge and other mantle domains, and are later involved in the subduction channel and orogenic system. The mantle-derived peridotites usually record complex metasomatism, ultra-high pressure (UHP) metamorphism and mantle-crust interaction during the orogenic processes. Zircons are rarely found in orogenic peridotites. These zircons in orogenic peridotites are generally formed during metasomatism, they can be divided into old zircons (mainly the cores of residual magmatic and recrystallized) and newly grown zircons. Three key factors for zircon formation in orogenic peridotites are that:(1) zircon has strong crystallization ability, and Zr is easier to exchange Si in zircon crystal structure than other elements in the mantle; (2) metamorphic destruction of Zr-bearing minerals and precipitation of intergranular melts during the high-grade metamorphism can nucleate zircon under sub-solidus conditions; (3) the melts/fluids released from the subducted crust can metasomatize the mantle wedge to form zircons. In-situ studies on zircons and zircon inclusions enclosed in mantle minerals indicate that zircon can be an ideal indicator for mantle-crust interaction in subduction zones. The inclusions in zircons and Hf-O isotope of zircons are effective to reflect the composition of the melts/fluids, source properties, and the physical and chemical conditions. Dating of the zircons has been widely used in the studies of lithospheric evolution and crust-mantle interaction. During the complex processes of plate convergence, the orogenic peridotites can be subjected to the melt/fluid metasomatism, modifying the mineral and elemental compositions of peridotites. Thus, zircon is very useful to unravel the history of specific lithospheric mantle and the relationship between continental cratonic cores and their margins.