Composition Of Continental Crust Research Articles

Layered Archean lower continental crust: Constraints from granulite terrains and xenoliths

Our understanding of crust–mantle differentiation is hampered by the large uncertainty in the composition of the inaccessible lower continental crust. The lower-crustal estimates derived from granulite terrains are compositionally much more evolved than those based on granulite xenoliths and there is no consensus on the origin of the compositional discrepancy. Here an integrated study of granulite terrains, granulite xenoliths and lower crustal–derived magmatic rocks from the Zhangjiakou region of the northern North China craton and a combination of geochronological, mineralogical, geochemical and geophysical data enable us to establish a complete Archean lower crustal cross-section for the first time. It shows that the late Archean lower crust beneath the North China craton is layered, with an intermediate upper lower crust and a mafic lowermost crust. The tonalite–trondhjemite–granodiorite (TTG)-dominated granulite terrains may merely represent the upper lower crust. Our results successfully solve the long-standing question regarding the compositional difference between granulite terrains and xenoliths. It is inferred that a layered Archean lower crust persists in many other cratons (e.g., the Siberian, Slave, Superior, and Tanzanian cratons) and it has an overall composition different from current lower crustal estimates. We further show that the Archean mafic lower crust is temporally coupled with the upper crust. This finding is significant as it raises an important question whether the mafic lower crust and exposed TTGs are genetically linked, which may be a key to understanding crust generation and differentiation processes.

Neoproterozoic sedimentary rocks in the Liantuo Formation in South China and their implication based on geochronological and Sr–Nd–Hf–Pb isotope insights

Deposition of the Liantuo Formation is closely associated with the evolutionary history of the Rodinia supercontinent, as indicated by the breakup of the Yangtze Craton in South China. In this paper, we also carried out a detailed UPb zircon examination of the Liantuo Formation. Radiometric dating of sandstone within the Liantuo Formation suggested that it was deposited ca. 790 Ma, which coordinated with the age-paleopole contradictions in the Liantuo Formation for exploring the evolutionary history of the Yangtze Block during the breakup of the Rodinia supercontinent. All the Liantuo sandstones and shales have variable Sr isotopic compositions, with initial Sr isotope ratios ranging from 0.6920 to 0.7217, which support significant fluvial contributions during the deposition of the Liantuo Formation. Samples of the Liantuo Formation show distinct variations in NdHf isotopic compositions, suggesting that the Nantuo Formation received relatively juvenile materials (e.g., the newly formed Neoproterozoic rocks from the northern margin of the Yangtze Block), whereas the Liantuo Formation received relatively mature inputs (e.g., the Huangling granitoids and/or Kongling complex). The samples from the Liantuo Formation also contain various Pb isotopes (206Pb/204Pb = 17.56–17.76, 207Pb/204Pb = 15.05–15.43 and 208Pb/204Pb = 35.51–36.79). These values are also close to the Pb isotopic compositions of the Kongling complex, which suggest that the sedimentary rocks were enriched (the Kongling complex) and are consistent with the NdHf isotope data. That is, the major provenance of the Liantuo Formation is from the South China Block basement, which is the local Kongling complex. In contrast, the Luoquan diamictite can represent the well-mixed composition of the upper continental crust (UCC) in the North China Block during the Neoproterozoic.

Tungsten stable isotope composition of the upper continental crust

Tungsten (W) stable isotope data for twenty-four composites made from the fine-grained matrix of glacial diamictites, deposited between the Mesoarchean and the Paleozoic, as well as two Archean tonalite-trondhjemite-granodiorites (TTG) are used to refine the W isotope compositions (expressed as δ186W deviations from the NIST 3163 W standard) of the upper continental crust (UCC). To do this we evaluate W isotope fractionation in the diamictites as the result of both magmatic and low-temperature processes. The δ186W values of the diamictites are heterogeneous (0.017 ± 0.011 ‰, 2SE to 0.182 ± 0.007 ‰, 2SE), encompassing the range of previously published values for igneous rocks. We find that δ186W correlates positively with the diamictite’s chemical index of alteration (CIA), a measure of the degree of chemical weathering in the provenance, suggesting that the continental regolith (i.e., surface material composed of weathered residues) is isotopically heavy for W. Since W in rivers and ocean water is also isotopically heavy, this suggests that equilibration with Fe-Mn-oxides controls the W isotope composition of surface waters and that such oxides are not important constituents of the regolith, whose W is likely accommodated in clays. Using diamictites with low CIA (<60, n = 9), we calculate an average δ186W of 0.046 ± 0.036 ‰ (2SD) for the UCC, from 2.3 to 0.3 Ga. This average is combined with the TTG data of this study, as well as data from the literature for upper crustal rocks to obtain a representative δ186W value of the UCC of 0.046 ± 0.046 ‰ (2SD). The W stable isotope composition of the UCC is lighter than that of mantle-derived melts (MORB and OIB, with an average δ186W = 0.082 ± 0.026 ‰, 2SD), and much lighter than the weighted average δ186W of intra-oceanic arcs (δ186W = 0.104 ± 0.052 ‰, 2SD), though the spread in the arc rocks is large. The significant difference in δ186W between average intra-oceanic island arc lavas and UCC is intriguing given that continental crust is thought to form primarily via arc magmatism. This difference suggests that not all modern intra-oceanic arcs are representative of those that formed the continental crust. Rather, only arcs that are enriched in incompatible trace elements, which also tend to have lighter W isotope compositions, may have been involved in making new continental crust. Alternatively, there may have been a secular change in the W isotope composition of arc magmas.

Andesitic arc magmas derived from two contrasting mélange origins: Evidence from central Tibetan dioritic porphyries

Numerous studies have argued that the andesitic model (i.e., the arc directly produces andesitic magmas) rather than the basaltic-input model (i.e., andesitic magmas were derived by fractionation from basaltic magmas) better explains the composition of continental crust. However, it remains controversial as to how andesitic magmas are directly produced in subduction zones, and this weakens the basis of the andesitic model. Recently, the mélange-diapir model has been increasingly considered to be an important mechanism for the direct generation of arc andesites, and would thus support the andesitic model for crustal growth. Moreover, melting experiments on mélange rocks show that varying the composition of mélange rocks (e.g., serpentinite-dominated and sediment-dominated mélange) can produce a range of primary andesitic arc magmas, from tholeiitic, calc-alkaline, to high-K calc-alkaline and shoshonitic magma types. However, it remains challenging to determine the different mélange sources for low-K to high-K arc andesites. Here we verify these experimental results using data from early Cretaceous dioritic porphyries in the southern Qiangtang block of central Tibetan plateau. The generation of these dioritic rocks cannot be linked to any coeval basaltic rocks in this region and so they may represent primary andesitic magmas. These dioritic porphyries are divided into low-K type-1 and high-K type-2. Type-1 has higher δ26Mg values (−0.16 to −0.08‰) than MORBs (mid-ocean ridge basalt, δ26Mg = −0.25 ± 0.06‰), which can be ascribed to the contribution of subducted bulk/un-differentiated serpentinite in their source. Combined with their low K contents and La/Sm ratios, and high NdHf isotope ratios (εNd(t) = 3.36–5.03; εHf(t) = 12.88–13.76), we suggest a serpentine-dominated mélange for their origin. The type-2 and type-1 rocks have similar trace-element distribution patterns, the same ages (∼124 Ma), and are indistinguishable in field outcrops, indicating their common petrogenesis. However, compared to type-1, type-2 has higher K and Th contents, and La/Sm and Th/Nd ratios with lower NdHf isotope values (εNd(t) = −1.61 to +0.30; εHf(t) = 3.78–4.39), and mantle-like δ26Mg values (−0.28 to −0.17‰). This likely indicates derivation from a sediment-dominated mélange for type-2. This study thus verifies the experimental results of melting of different mélange from natural rock record and shows that the mélange model provides an important mechanism for generating variable andesitic arc magmas and so the formation of the andesitic continental crust.

Nd isotope and element composition of shales from the Parnaíba Basin, NE Brazil: Insights for provenance shifts in a cratonic basin

Sm-Nd and Sr isotope compositions of 24 samples of fine-grained clastic rocks from eastern Parnaíba basin, NE-Brazil, are presented along with major and trace element compositions of 19 of these samples. Samples were collected from outcrops of the Paleozoic Canindé (Pimenteiras, Longá and Poti) and Balsas (Piaui Formation) groups, and from the Jurassic Pastos Bons Formation. Discrimination diagrams based on Th, Sc, La and Zr indicate sedimentary provenance from felsic dominant sources and high degrees of crustal recycling. PAAS-normalized REE diagrams show that the Pimenteiras and Longá Formations display flat patterns with slight REE enrichment, and initial εNd and 87Sr/86Sr around the average composition of the upper continental crust, respectively between −8.9 and −14.6 and between 0.711 and 0.760, indicating provenance from old Precambrian basement. The Nd TDM model ages of the Pimenteiras between 1.32 Ga and 2.00 Ga indicate heterogeneity of source areas during the Devonian, and TDM ages around 1.6 Ga in the Longá Formation samples suggest intense recycling of older sediments. The Poti formation samples yield TDM ages of 2.30 Ga and 2.32 Ga, respectively, suggesting tectonic reactivation and new input from Archean-Paleoproterozoic basement complexes. The Pastos Bons Formation samples show lower Th-Sc ratios, light-REE depletion, and less negative initial εNd, indicating contribution from mafic rocks, probably from the underlying Mosquito Formation intraplate tholeiitic basalts and dolerites.

Mercury abundance and isotopic composition in granitic rocks: Implications for Hg cycling in the upper continental crust

Mercury (Hg) is a highly volatile metal exhibiting both isotope mass-dependent (MDF, defined as δ202Hg) and mass-independent (MIF, expressed as Δ199Hg) fractionation. Assessing the Hg isotopic composition of the upper continental crust is crucial for the use of this new geochemical tool for petrogenetic tracing, particularly for understanding crust-mantle interaction. Here, we report Hg isotopic compositions of granitic rocks with distinct source affinity (I-, A-, and S-type) and of metasedimentary enclaves from the South China Craton. The results of more than 100 rock samples show large δ202Hg variations of −2.03 to 0.47 ‰ and small Δ199Hg variations of −0.16 to 0.09 ‰. The S-type granites studied here exhibit higher δ202Hg values (−0.58 to 0.00 ‰; n = 57) than their sedimentary protoliths (metasedimentary enclaves; −2.03 to −0.91 ‰; n = 7), suggesting significant Hg-MDF during magmatic processes. The Δ199Hg values of S-type granites (−0.08 to 0.09 ‰) are indistinguishable from their sedimentary protoliths (−0.16 to 0.02 ‰), suggestive of the absence of Hg-MIF during crustal anatexis and subsequent magmatic differentiation. The I- and A-type granites studied have smaller Δ199Hg variations of −0.11 to 0.04 ‰ (n = 28) and −0.07 to 0.04 ‰ (n = 15), respectively, but are within the range of the S-type granites. The compositional range of all granite variants may be explained by the variable intensity of large-scale MASH (melting-assimilation-storage-homogenization) processes. Our new data combined with previously published data of igneous, sedimentary, and metamorphic rocks allow us to estimate that the upper continental crust (UCC) has a Hg abundance of 13.5 ppb and a weighted average δ202Hg of −1.15 ± 1.01 ‰ (2SD), both of which are higher than the corresponding values of the primitive mantle. However, the weighted average Δ199Hg value of the UCC (0.03 ± 0.15 ‰; 2SD) is nondistinguished from the primitive mantle, suggesting no obvious Hg-MIF during the formation and differentiation of the continental crust.

Formation of lower arc crust by magmatic underplating revealed by high-precision geochronology

Abstract In modern plate tectonic regimes, continental crust is generated above subduction zones in magmatic island arcs. Models for continental growth—largely based on the modeling of geochemical processes that can transform mantle melts into the intermediate composition of bulk continental crust—have been hampered by a lack of definitive geochronology, which could clarify the temporal emplacement of igneous rocks at the base of island arc crust. The Kohistan Arc Complex (KAC) of Pakistan is a rare window into a nearly complete section of lower arc crust, revealing a sequence of igneous rocks representing discrete batches of magma underplated along the base of the arc crust. We present high-precision isotope dilution–thermal ionization mass spectrometry U-Pb geochronological data from zircon, and Lu-Hf and Sm-Nd isochron data from several mafic lower crustal cumulates within the KAC. These data establish a clear downward-younging age trend throughout this succession of cumulate complexes, demonstrating a total magmatic duration of ~20 m.y., corresponding to a rate of lower crust formation of up to 200–260 km3 km–1 Ma–1. These results provide the first direct evidence of a sequential process of underplating, revealing a fundamental mechanism responsible for building the root of island arc crust.

Zircon U–Pb ages and geochemistry of deep-seated crustal xenoliths from Yangyuan: Implications for the evolution of lower crust beneath the Trans North China Orogen

Deep-seated crustal xenoliths captured by volcanic rocks play a rare opportunity in understanding the composition and evolution of the lower continental crust. Here, we present mineral chemistry, whole-rock major and trace elements and Sr–Nd isotopes, and in-situ zircon U–Pb ages and Hf isotope data for a suite of granulite and biotite gneiss xenoliths entrained in the Oligocene basalts from the Yangyuan county in the central part of the Trans North China Orogen (TNCO) with aims to decipher the composition and evolutionary history of the lower crust beneath this area. Calculated equilibrium temperatures (841–933 °C) and pressures (9.7–14.3 kbar) suggest an origin of the Yangyuan granulites from the lower crust. The dominantly Archean–Proterozoic zircon ages and low Mg# values (29.0–68.4) of the Yangyuan crustal xenoliths indicate that they represent the ancient crust and the fractional crystallization plays a key role during the protolith formation. Furthermore, the zircon U–Pb–Hf isotopes data reveal multiple thermal events, including two main peaks of crustal growth at ∼2.7 Ga and ∼2.5 Ga, ∼1.85–1.80 Ga and the Phanerozoic accretion and reworking of the lower crust beneath Yangyuan. In addition, a comparison study of crustal xenoliths from Yangyuan, Nangaoya and Hannuoba areas demonstrates that the lower crust beneath the central–northern part of the TNCO experienced as a whole the Neoarchean crustal growth, Paleoproterozoic metamorphism and Phanerozoic magmatic underplating. More importantly, the absence of U–Pb age peak of ∼1.95 Ga and Hf model age peaks of ∼3.9–3.8 Ga, ∼2.9–2.8 Ga, ∼2.2–2.1 Ga and ∼700–600 Ma in Yangyuan shows a heterogeneous evolution of the lower crust beneath the TNCO, which could pave the way for a more comprehensive picture of the crustal evolutional history beneath the North China Craton.

Tracing the Early Crustal Evolution of the North China Craton: New Constraints from the Geochronology and Hf Isotopes of Fuchsite Quartzite in the Lulong Area, Eastern Hebei Province

Understanding the composition, formation and evolution of the oldest continental crust is crucial for comprehending the mechanism and timing of crustal growth and differentiation on early Earth. However, the preservation of the ancient continental crust is limited due to extensive reworking by later tectonothermal events. In the Lulong area of eastern Hebei, abundant ca. 3.8–3.4 Ga detrital zircons of the fuchsite quartzite have been previously identified. Nonetheless, the provenance and Hf isotopic compositions of the fuchsite quartzite remain unclear. In this study, we present new detrital zircon ages and Hf isotopic for the fuchsite quartzite in the Lulong area to establish the timing of deposition, the provenance and the regional stratigraphic relationship. Zircon U-Pb dating indicates that the fuchsite quartzite was deposited between 3.3–3.1 Ga and most grains were sourced from the 3.8 Ga TTG gneisses and Paleoarchean magmas. Field investigations and regional correlations reveal that the fuchsite quartzite from the Lulong area is equivalent to that of the Caozhuang area. Zircon Hf isotopic data from eastern Hebei Province (Lulong and Caozhuang areas) and Anshan and Xinyang areas indicate that the oldest crustal growth event of North China Craton occurred in the Hadean.

Cerium geochemical composition of the upper continental crust through time: Implications for tracing past surface redox conditions

To investigate the use of cerium (Ce) in the upper continental crust (UCC) for tracing oxidative weathering, we measured the Ce anomaly (Ce/Ce*), the Ce stable isotope composition (δ142Ce), and the average Ce valence state (Ce#) in twenty-four glacial diamictite composites with depositional ages ranging from ∼2.9 to 0.3 Ga. We observe no distinguishable secular changes in Ce/Ce* (Ce/Ce* = 0.87–0.97) or δ142Ce (−0.092 ± 0.033 to 0.028 ± 0.043‰, 2 S.D.) of the diamictites. Limited elemental partitioning and isotopic fractionation of Ce can be ascribed to the immobilization of REEs, including Ce, in the diamictites during oxidative weathering. We calculated the present-day UCC δ142Ce to be −0.025 (median) ± 0.018‰ (MAD) or −0.029 (mean) ± 0.058‰ (2 S.D.). We assigned average valence state data of Ce (Ce#) in the diamictites into three age groups relative to the timing of the Great Oxidation Event (GOE): pre-GOE (2.9–2.4 Ga; Ce# = 3.09 to 3.22; average of 3.14), syn-GOE (2.4–2.2 Ga; Ce# = 3.18 to 3.73; average of 3.29), and post-GOE ages (0.8–0.3 Ga; Ce# = 3.21 to 3.68; average of 3.36). Our results show distinguishable correlations between Ce# and common weathering (major element ratios and Li isotopes) and redox tracers (e.g., Mn, Mo, V and Th/U) due to increasingly oxidative weathering with time. These results suggest that the valence state of Ce is a sensitive indicator of redox, and advise against relying solely on element- or isotope-based proxies. Combining redox proxies with valence state information can strengthen and complement the interpretation of past redox conditions.

Deep formation of Earth’s earliest continental crust consistent with subduction

About four billion years ago, Earth’s outer layer is thought to have been composed mostly of a 25- to 50-km-thick basaltic crust that differentiated to form the oldest stable continental crust. However, the tectonic processes responsible for the formation of this continental material remain controversial. Suggested explanations include convergent plate boundary processes akin to subduction operating today and a variety of relatively shallow (<50 km) non-plate-tectonic intracrustal mechanisms. Here we perform high-pressure–temperature melting experiments on an oceanic plateau analogue for the early basaltic crust and show that magmas with the composition of the early continental crust cannot form at pressures <1.4 GPa (~50 km depth). This suggests that Eoarchaean continental magmas are formed in deep (>50 km) subduction-like environments. Our results support previous Eoarchaean field evidence and analyses of igneous rocks that date to 4.0–3.6 billion years ago, which are consistent with subduction-like processes and suggest a primitive type of plate tectonics operated as long as 4 billion years ago on early Earth.

The Mineral Chemistry of Magnetite and Its Constraints on Ore-Forming Mechanism in the Sandaozhuang Skarn-Type W-Mo Deposit in East Qinling, China

The Sandaozhuang super-large W-Mo deposit is located in the southern margin of the North China Craton, within the well-known East Qinling Mo mineralization belt, and is one of the typical skarn-type W-Mo deposits in China. Based on EMPA and LA-ICP-MS analyses, major and trace elements were presented, and the mineral chemistry of magnetite at various mineralization stages was discussed. Combining field observations, petrography, and geochemical characteristics, the magnetite at the Sandaozhuang deposit can be classified into three types, namely early-magmatic-stage high-temperature magnetite (Mag1), potassic-alteration-stage magnetite (Mag2), and retrograde-alteration-stage magnetite (Mag3). The Mag1 and Mag2 magnetites primarily occurred in granites in association with potassium (K) feldspar and biotite, whereas Mag3 is associated with metallic sulfide minerals that occurred mainly in vein-like structures in skarn. The three magnetites Mag1, Mag2, and Mag3 can be distinguished as having magmatic, magmatic–hydrothermal transition, and hydrothermal origins, respectively. All three types of magnetite exhibit a depletion of high-field-strength elements (HFSEs) such as Zr, Hf, Nb, Ta, and Ti, and large-ion lithophile elements (LILEs) including Rb, K, Ba, and Sr, compared to the mean continental crust composition. Conversely, they are enriched in elements such as Sn, Mo, V, Cr, Zn, and Ga. Mag3 showed no significant depletion of Co, Ni, Cu, and Bi, indicating that the influence of coexisting sulfides on the composition of magnetite at the Sandaozhuang deposit is limited. There are systematic variations in major and trace elements from Mag1 to Mag3, which exhibited similar patterns in trace element spider and rare earth element diagrams, and Y/Ho ratio, indicating a consistent source for the three types of magnetite. The changes in V and Cr contents and (Ti + V) vs. (Al + Mn) diagram of magnetite at the Sandaozhuang deposit reflected the evolution of ore-forming fluids with an initial increase in oxygen fugacity and a subsequent decrease, as well as a gradual decrease in temperature during skarn mineralization. The early high-temperature and high-oxygen-fugacity magmatic fluids became W and Mo enriched by hydrothermal fluid interaction. The rapid change in fluid properties during the retrograde alteration stage led to the precipitation of scheelite and molybdenite.

Seismic Evidence for Metamorphic Densification of the Lower Continental Crust in Eastern North America

AbstractThe composition of the lower continental crust, as well as its formation, growth, and evolution, remains a fundamental subject to be understood. In this study, we carry out a comparative and integrative analysis of seismic tomographic models, teleseismic receiver function results, and Airy isostasy in order to investigate the properties of the lower continental crust in eastern North America. We extract the depths for Vs of 4.0 km/s, 4.2 km/s, and 4.5 km/s from three selected tomographic models and calculate the differences between the Vs depth contours and the Moho depth defined by receiver functions. We then calculate the Airy isostatic Moho depth and its misfit with the receiver‐function‐defined Moho. Our analysis reveals three key features: (a) the deepening of the Vs depth contours and the strong negative Airy misfit within the U.S. Grenville Province; (b) a seismically faster‐than‐average and compositionally denser‐than‐average lowermost crust in the eastern North American Craton and the Grenville Province; and (c) the thickest, seismically fastest, and densest lowermost crust beneath the southern Grenville Front, the southern Grenville‐Appalachian boundary, and the U.S.‐Canada national border. We suggest that the lower crust of the craton and the Grenville Province has densified through garnet‐forming metamorphic reactions during and after orogenesis, contributing to the widely distributed fast‐velocity layer. The lower crust beneath the tectonic boundaries could have experienced more extensive garnet growth during orogenesis and emplacement of mafic magma. This study provides new constraints on the seismic and compositional properties of the lower crust in eastern North America.

Geophysical‐Geochemical Modeling of Deep Crustal Compositions: Examples of Continental Crust in Typical Tectonic Settings and North China Craton

AbstractThe chemical composition of the deep continental crust is key to understanding the formation and evolution of the continental crust. Constraining the chemical composition of present‐day deep continental crust is, however, limited by indirect accessibility. This paper presents a modeling method for constraining deep crustal chemical structures from observed crustal seismic structures. We compiled a set of published composition models for the continental crust to construct functional relationships between seismic wave speed and major oxide content in the crust. Phase equilibria and compressional wave speeds (VP) for each composition model were calculated over a range of depths and temperatures of the deep crust. For conditions within the alpha(α)‐quartz stability field, robust functional relationships were obtained between VP and major oxide contents of the crust. Based on these relationships, observed VP of the deep crust can be inverted to chemical compositions for regions with given geotherms. We provide a MATLAB code for this process (CalcCrustComp). We apply this method to constrain compositions from deep crustal VP of global typical tectonic settings and the North China Craton (NCC). Our modeling results suggest that the lower crust in subduction‐related and rifting‐related tectonic settings may be more mafic than platforms/shields and orogens. The low VP signature in the deep crust of the NCC can be explained by intermediate crustal compositions, higher water contents, and/or higher temperatures. The chemical structure obtained by this method can serve as a reference model to further identify deep crustal features.

Phosphorus deficit in continental crust induced by recycling of apatite-bearing cumulates

Abstract Trace-element anomalies in the continental crust (e.g., Nb-Ta-Pb) are useful to decipher the formation and evolution of continents. We found that the trace-element patterns of upper, middle, and lower crust are all marked by strongly negative phosphorus anomalies, which were not likely to have been generated directly by mantle melting. In order to address this “crust composition paradox,” we compiled major- and trace-element data from global arc magmas, igneous minerals, and deep arc cumulates and found that the phosphorus deficit in continental crust is coupled with an elevated Th/La ratio. This feature can be best explained by the fractional crystallization of apatite during arc magma evolution rather than the influence of a subducted slab. Accumulation of apatite-bearing mafic cumulates in deep arcs followed by foundering into the upper mantle causes the mismatch in the phosphorus systematics between the mantle and the continental crust. This process plays a critical role in shaping the chemical composition of continental crust.

Andesites and evolution of the continental crust: Perspectives from the Central Volcanic Zone of the Andes

Named for the Andes, andesites (53%–63% SiO2) are the archetypal magma erupted at magmatic arcs. They have been established as the average composition of continental crust and as such are integral to the growth and evolution of the continental crust. However, andesites are quite variable in trace element and isotopic composition reflecting disparate paths of origin. Herein we return to the original site of their identification, the Central Andes, and use a comprehensive dataset of published and unpublished trace elements and isotopes to show that during the past 6 Myr two distinct types of andesite have erupted in the Central Volcanic Zone (CVZ), which correspond with different geodynamic conditions. Consistent with previous work, we confirm that major composite cones and minor centers of the steady state (low magmatic flux) Quaternary CVZ arc have trace element and isotopic characteristics consistent with magma generation/fractionation in the lower crust. Within the Quaternary arc centers, there are also significant latitudinal variations that correspond with the age, composition, and P-T conditions of the lower crust. However, in contrast to this prevailing model, in the 21–24°S segment 6–1 Ma andesites from ignimbrites and lava domes associated with the peak of the regional Neogene ignimbrite flare-up have compositions that indicate these andesites are hybrids between mantle-derived basalts and upper crustal lithologies. Since ∼1 Ma, andesites in young silicic lava domes associated with the regional flare-up are compositionally indistinguishable from proximal Quaternary arc centers, indicating a return to steady-state magmatism and lower crustal production of andesites. We propose that the transition from upper crustal to lower crustal andesite production results from a decrease in mantle heat input and subsequent relaxation of the regional geotherm during the waning of the flare-up event. The two modes of andesite production have significant implications for the production and evolution of the CVZ arc crust. During the flare-up, prodigious amounts of basalt were emplaced into the mid-crust, resulting in the production of large volumes of hybrid intermediate magmas in the mid and upper crust. In contrast, the lower crustal differentiation recorded in the Quaternary steady state arc andesites would result in the formation of a dense crystalline residue in the lower crust and an overall densification of the lower crust. Over time, gravity instabilities associated with this densification may ultimately aid in the delamination of the dense lower crustal root, triggering flare-ups. These differences in andesite production may help explain the cyclicity (flare-up cycles) observed in mature continental arcs and emphasizes that andesite is not a monotonous composition and can vary with depth-dependent intra-crustal differentiation related to magmatic flux.

ТИПЫ ЗОЛОТО-ПОЛИМЕТАЛЛЬНОЙ МИНЕРАЛИЗАЦИИ В ПОРОДАХ ДОКЕМБРИЯ НИМНЫРСКОГО ТЕРРЕЙНА АЛДАНО-СТАНОВОГО ЩИТА (ЮГ СЕВЕРО-АЗИАТСКОГО КРАТОНА)

The study of material composition of the Early Precambrian continental earth crust contributes to the identification of its metallogeny. Deposits of graphite, phlogopite, and iron have been discovered in the Paleoproterozoic granulite complexes of the Aldan-Stanovoy Shield on the southern margin of the North Asian Craton, and the first gold deposit has recently been discovered. Deposits are confined to the extended boundary of the infra- and supracrustal complexes within which multistage deformations, intense metasomatosis and metamorphism of various facies are observed. Modern methods of microanalysis allow scientists to significantly supplement data on the mineral composition of deposits, characterize conditions of their formation and make a comparison. The paper considers the geological structure of granulite complexes of the central part of the Aldan-Stanovoy Shield and ore mineralization in graphite-bearing quartzites, phlogopite-containing skarns, gold-bearing crystalline shales and diaphthorites. Ore and wallrock mineral associations have been identified. The comparison was made with well-studied ore deposits near the border of the Karelian Craton. The types of mineralization and directions of comparative metallogenic analysis are discussed.

Composition and Evolution of Continental Crust at Orogenic Belts: Constraints From a 3‐D Crustal Model of Southeast China

AbstractThe chemical composition of continental crust is fundamental and crucial to understanding its origin and evolution. Orogenic belts make up about 30% of the global continental crust, yet their chemical compositions are still poorly constrained. In this study, we report an overall estimate for the continental crust composition of a collisional orogenic belt terrane: the Southeast China. The estimate is obtained through a 3‐D crustal model combining a lithology proportion model and probability density function models for element concentrations of various lithologies. Based on our model, the Southeast China crust is amongst one of the most evolved continental crust composition estimates (71.0% SiO2 content for the upper crust and 64.7% SiO2 for the bulk crust). A strong correlation between SiO2 and the chemical index of alteration (CIA) of different composition models for upper continental crust (UCC) indicates that chemical weathering is a controlling factor for the development of a felsic UCC. In contrast, weak correlations of SiO2 with Ninorm (15.9*Ni/Al2O3) and CIA suggest that igneous differentiation and chemical weathering act together to modify the composition of the bulk continental crust. Orogenic belt terranes like the Southeast China can develop a highly evolved crustal composition, especially for the upper crust, due to extensive intracrustal reworking and differentiation, enhanced chemical weathering, and potential subsequent delamination and removal of thickened crust root. Therefore, an overall estimation of the average composition of continental crust should take into account the more evolved composition of orogenic belts.

Nb-Ta fractionation by amphibole and biotite during magmatic evolution: Implications for the low Nb/Ta ratios of continental crust

How did the low niobium/tantalum (Nb/Ta) ratios of continental crust form is a hotly debated issue? Here, we report the Nb Ta features of a well-characterized magmatic rock series, mainly composed of amphibole and biotite cumulates, from the Northern Dabie orogenic belt in China. The Nb/Ta ratios of whole-rock (19.5–26.2), amphibole (16.8–41.1), and biotite (21.1–49.6) are negatively correlated with their Mg # values, indicating that the separation of amphibole and biotite could not only cause Nb Ta fractionation but also reduce the Nb/Ta ratios of residual magmas during magmatic evolution. Furthermore, we compile the Nb/Ta ratios of global arc magmas ( n = 15,696) to examine the role of amphibole and biotite in the differentiation of arc magmas. The Nb/Ta ratios of global arc magmas display a negative correlation with SiO 2 contents and positive correlations with Dy/Yb and Ba/Rb ratios. Additionally, the compositions of the upper continental crust (UCC) exactly fall on the evolution line of Nb/Ta vs. SiO 2 and Nb/Ta vs. Ba/Rb, yet significantly falls below the evolution line of Nb/Ta vs. Dy/Yb. These observations show that amphibole and biotite are two important magmatic phases necessary for processes of arc magma differentiation and that they play a role together in the formation of low Nb/Ta ratios of the continental crust. As a whole, we suggest that amphibole- and biotite-rich rocks stored in the deep crust are one of the potential hidden reservoirs with higher Nb/Ta ratios, which may contribute to the depleted Nb during the formation process of continental crust. • Nb Ta fractionation of arc magmas could be induced by amphibole and biotite. • Amphibole- and biotite-rich rocks in deep crust are potential high Nb/Ta reservoirs.

Geochemistry of Triassic metasediments from easternmost part of Sakar Unit, Sakar-Strandzha Zone, SE Bulgaria

Triassic metasediments from the easternmost part of Sakar Unit, Sakar-Strandzha Zone, SE Bulgaria, show a high variety in their mineral and chemical composition from shales, wakes, litharenites to arkoses. Geochemical signatures suggest low to moderate weathering degree at their source and upper continental crust composition. The Triassic metasedimentary rocks were derived from felsic to intermediate igneous rocks with some recycled sediment input. On tectonic setting discrimination diagrams, based on immobile trace elements, the studied metasediments fall in the fields of continental island arc and active continental margin.