Miocene adakites in south Tibet: Partial melting of the thickened Lhasa juvenile mafic lower crust with the involvement of ancient Indian continental crust compositions

2019 ◽  
Vol 132 (5-6) ◽  
pp. 1273-1290
Author(s):  
Haoyu Yan ◽  
Xiaoping Long ◽  
Jie Li ◽  
Qiang Wang ◽  
Xuan-Ce Wang ◽  
...  
Keyword(s):  
Rare Earth ◽  
Rare Earth Elements ◽  
Partial Melting ◽  
Continental Crust ◽  
Lower Crust ◽  
Adakitic Rocks ◽  
Magma Sources ◽  
Mafic Lower Crust ◽  
Porphyritic Texture ◽  
South Tibet

Abstract Although postcollisional adakitic rocks are widely distributed in the southern Lhasa subterrane, their petrogenesis remains controversial. Complex petrogenesis models, mainly including partial melting of subducted oceanic crust, partial melting of the Indian lower continental crust, and magma mixing, are pivotal in reconstruction of the postcollisional dynamic processes in south Tibet. In order to constrain the geodynamic processes, we present systemic geochronological and geochemical data for newly discovered adakitic dikes in the Xigaze area, southern Lhasa subterrane. Based on the K2O and Na2O contents, the Xigaze dikes can be divided into K-rich and Na-rich dikes. Zircon U-Pb dating for the Xigaze K- and Na-rich dikes yielded ages of ca. 10.31 Ma and 14.78–12.75 Ma, respectively. The K-rich dikes show porphyritic texture and are characterized by high SiO2 (68.91–69.59 wt%) and K2O (5.53–5.68 wt%) contents and low Na2O/K2O (0.48–0.60) ratios, with Al2O3/(CaO + Na2O + K2O) (=A/CNK) ratios of 1.07–1.23. They have lower MgO (0.63–0.64 wt%), Mg# (37–39), and Cr (18.56–26.62 ppm) and Ni (4.37–4.62) contents. In addition, the K-rich dikes display enriched ([La/Yb]N = 65–68) light rare earth elements (LREEs), low concentrations of heavy rare earth elements (HREEs) and Y (e.g., Yb = 0.83–0.86 ppm; Y = 10.56–11.55 ppm), and high Sr (841–923 ppm), with high Sr/Y (74–84) ratios, indicating geochemical characteristics of typical adakitic rocks. Compared with the K-rich dikes, the Na-rich dikes also display porphyritic texture, but they have lower SiO2 (59.14–64.87 wt%) and K2O (1.98–3.25 wt%) contents, and higher Na2O (4.43–5.64 wt%) and MgO (1.40–3.08 wt%) contents, Mg# (46–59), and Cr (22.62–82.93 ppm) and Ni (8.91–39.76 ppm) contents. The HREE abundances (e.g., Yb = 0.36–0.81 ppm; Y = 5.30–10.56 ppm) of the Na-rich dikes are generally lower than the K-rich dikes. These Na-rich dikes are also characterized by adakitic geochemical features with high Sr/Y (60–223) but low (La/Yb)N (15–40) ratios. Both the K-rich and Na-rich dikes display distinct whole-rock-element geochemistry and Sr-Nd isotopic composition, with (87Sr/86Sr)i = 0.7121, εNd(t) = –8.62 to –8.11 and (87Sr/86Sr)i = 0.7054–0.7086, εNd(t) = –7.55 to –1.23 for K-rich and Na-rich dikes, respectively, which indicate different magma sources for the two types of dikes. The K-rich dikes were most likely derived from partial melts of Lhasa juvenile mafic lower crust with significant involvement of Indian continental crust compositions, whereas the Na-rich dikes were generated in the same way with less input of Indian continental crust compositions. Moreover, the postcollisional adakites in the southern Lhasa subterrane display distinctive spatial variations in geochemistry along the strike of this subterrane, indicating that the magma sources were heterogeneous. In combination with previously published data, we therefore suggest that all these late Oligocene to Miocene adakitic rocks were most likely generated dominantly by partial melting of the Lhasa mafic lower crust with involvement of Indian continental crust components, which was probably triggered by the tearing of the subducting Indian plate.

scholarly journals Petrogenesis of the Early Cretaceous Aolunhua Adakitic Monzogranite Porphyries, Southern Great Xing’an Range, NE China: Implication for Geodynamic Setting of Mo Mineralization

Minerals ◽  
2020 ◽  
Vol 10 (4) ◽  
pp. 332
Author(s):  
Xiaohu He ◽  
Shucheng Tan ◽  
Zheng Liu ◽  
Zhongjie Bai ◽  
Xuance Wang ◽  
...  
Keyword(s):  
Partial Melting ◽  
Continental Crust ◽  
Early Cretaceous ◽  
Lower Crust ◽  
Evolutionary History ◽  
Ne China ◽  
Lower Continental Crust ◽  
Adakitic Rocks ◽  
Great Xing’An Range ◽  
Mafic Lower Crust

This paper reports on whole-rock major- and trace-elemental and Sr–Nd isotopic compositions of the Aolunhua adakitic monzogranite porphyries from the Xilamulun district in the southern Great Xing’an Range, Northeast (NE) China. The high-K calc-alkaline Aolunhua monzogranite porphyries are characterized by high Sr/Y ratios (34.59–91.02), Sr (362–809 ppm), and low Y contents (7.66–10.5 ppm), respectively. These rocks also show slightly enriched Sr and Nd isotopes ((87Sr/86Sr)i = 0.7051–0.7058; εNd(t) = −2.98–0.92), with young two-stage model ages (T2DM = 0.84–1.16 Ga). Such a signature indicates that these rocks were most likely formed by partial melting of juvenile mafic lower crust. Based on equilibrium melting and batch-melting equations, we performed incompatible trace elements modeling. Low FeOT/(FeOT + MgO) values indirectly reflect these adakitic rocks were derived from an oxidizing source related to magnesian granitoids. The decreasing content of TiO2, Fe2O3, Nb/Ta ratio, and moderately negative Eu anomalies suggest that minimal fractionation of Fe–Ti oxides and plagioclase may have occurred in their evolutionary history. The result shows that the Aolunhua adakitic porphyries and coeval adakitic intrusive rocks in this area had not experienced extensive fractional crystallization and were derived from 20%–40% partial melting of lower continental crust, which was composed of ~25%–40% and 5%–20% garnet-bearing amphibolite, respectively. Integrating with rock assemblages and regional tectonic evolutionary history in this regime, high (Sm/Yb)SN (SN—source normalized data, normalized to mafic lower continental crust with Yb = 1.5 ppm and Sm/Yb = 1.87 for continental adakite) and low YbSN ratios suggest that these rocks were generated in an extensional environment related to lithospheric delamination without crustal thickening. The collision between North China and Siberian cratons around 160 Ma blocked the westward movement of the lithosphere as a result of the subduction of Pacific plate, which then led to lithospheric delamination induced by asthenospheric upwelling and underplating. Subsequently, partial melting of mafic lower crust caused by mantle upwelling resulted in the Early Cretaceous magmatic activities of adakitic rocks and associated Mo mineralization in the southern Great Xing’an Range.

The composition and evolution of the continental crust: rare earth element evidence from sedimentary rocks

1981 ◽  
Vol 301 (1461) ◽  
pp. 381-399 ◽  
Keyword(s):  
Rare Earth ◽  
Partial Melting ◽  
Continental Crust ◽  
Rare Earth Element ◽  
Lower Crust ◽  
Sedimentary Rock ◽  
Earth Element ◽  
Upper Crust ◽  
Crustal Composition ◽  
Eu Anomaly

The composition of the present-day upper crust, inferred from the uniformity of sedimentary rock r.e.e. (rare earth element) patterns, is close to that of granodiorite. A revised ‘andesite’ model is used to obtain total crustal composition. The lower crust is the composition remaining, assuming that the upper crust, one-third of the total, is derived from intracrustal partial melting. The upper-crustal r.e.e. pattern has pronounced Eu depletion (Eu/Eu* = 0.64), the lower-crustal pattern has Eu enrichment (Eu/Eu* = 1.17) and the total crust has no Eu anomaly relative to chondritic abundances. The Eu depletion in the upper crust is attributed to retention of Eu in plagioclase in the lower crust. Because plagioclase is not stable below 40 km (> 10 kbar), the anomaly is intracrustal in origin. The Archaean upper crust has a different r.e.e. pattern to that of the present-day upper crust, being lower in total r.e.e., and La/Yb ratios, and lacking an Eu anomaly. These data are used to infer the Archaean upper-crustal composition, which resembles that of the present-day total crust, except that Ni and Cr contents are higher. The Archaean crustal composition can be modelled by a mixture of tholeiites and tonalite trondhjemites. The latter have steep light r.e.e.-enriched-heavy r.e.e.-depleted patterns, consistent with equilibration with garnet and hence probable mantle derivation. There is little reason to suppose that the Archaean lower crust was different in composition from the upper crust, except locally where partial melting episodes occurred. The r.e.e. evidence is consistent with isotopic and geological evidence for a low continental growth rate in the early Archaean, a massive increase (to about 70% of the total crust) between about 3000 and 2500 Ma B.P. and a slow increase until the present day. The change from Archaean to post-Archaean r.e.e. patterns in the upper crust is not isochronous, but is reflected in the sedimentary rock r.e.e. patterns at differing times in different continents. On the basis of a model composition for the mantle, 36% of the potassium, 30% of uranium, 15% of lanthanum and 3 % of ytterbium are concentrated in the present continental crust. This enrichment is related to ionic size and valency differences from common mantle cations (e.g. Mg, Fe). Pre-3.9 Ga B.P. crusts were obliterated by meteorite bombardment. No geochemical evidence exists for primordial anorthositic, sialic or mafic crusts.

Mesozoic adakitic rocks from the Xuzhou-Suzhou area, eastern China: Evidence for partial melting of delaminated lower continental crust

2006 ◽  
Vol 27 (4) ◽  
pp. 454-464 ◽  
Author(s):  
Wen-Liang Xu ◽  
Qing-Hai Wang ◽  
Dong-Yan Wang ◽  
Jing-Hui Guo ◽  
Fu-Ping Pei
Keyword(s):  
Partial Melting ◽  
Continental Crust ◽  
Eastern China ◽  
Lower Continental Crust ◽  
Adakitic Rocks

Petrogenesis of the Upper Cretaceous volcanism in the Kefken region, Western Pontides, NW Turkey

2021 ◽  
Author(s):  
Turgut Duzman ◽  
Ezgi Sağlam ◽  
Aral I. Okay
Keyword(s):  
Rare Earth ◽  
Rare Earth Elements ◽  
Partial Melting ◽  
Upper Cretaceous ◽  
Volcanic Rocks ◽  
Early Eocene ◽  
Heavy Rare Earth ◽  
Heavy Rare Earth Elements ◽  
Cretaceous Volcanism ◽  
Oceanic Slab

<p>The Upper Cretaceous volcanic and volcaniclastic rocks crop out along the Black Sea coastline in Turkey. They are part of a magmatic arc that formed as a result of northward subduction of the Tethys ocean beneath the southern margin of Laurasia. The lower part of the Upper Cretaceous volcanism in the Kefken region, 100 km northeast of Istanbul, is represented by basaltic andesites, andesites, agglomerates and tuffs, which have yielded Late Cretaceous (Campanian, ca. 83 Ma) U-Pb zircon ages. The volcanic and volcanoclastic rocks are stratigraphically overlain by shallow to deep marine limestones, which range in age from Late Campanian to Early Eocene.  Geochemically, basaltic andesites and andesites display negative anomalies in Nb, Ta and Ti, enrichment in large ion lithophile elements (LILE) relative to high field strength elements (HFSE). Light rare earth elements (LREE) show slightly enrichment relative to heavy rare earth elements (La<sub>cn</sub>/Yb<sub>cn</sub> =2.51-3.63) and there are slight negative Eu anomalies (Eu/Eu* = 0.71-0.95) in basaltic andesite and andesite samples. The geochemical data indicate that Campanian volcanic rocks were derived from the partial melting of the mantle wedge induced by hydrous fluids released by dehydration of the subducted oceanic slab.</p><p>There is also a horizon of volcanic rocks, about 230 m thick, within the Late Campanian-Early Eocene limestone sequence.  This volcanic horizon, which consists of pillow basalts, porphyritic basalts,  andesites and dacites, is of Maastrichtian age based on paleontological data from the intra-pillow sediments and U-Pb zircon ages from the andesites and dacites (72-68 Ma).  The Maastrichtian andesites and dacites are geochemically distinct from the Campanian volcanic rocks. They show distinct adakite-like geochemical signatures with high ratios of Sr/Y (>85.5), high La<sub>cn</sub>/Yb<sub>cn </sub>(16.4-23.7) ratios, low content of Y (7.4-8.6 ppm) and low content of heavy rare-earth elements (HREE). The adakitic rocks most probably formed as a result of partial melting of the subducting oceanic slab under garnet and amphibole stable conditions.</p><p>The Upper Cretaceous arc sequence in the Kefken region shows a change from typical subduction-related magmas to adakitic ones, accompanied by decrease in the volcanism.</p><p> </p><p> </p>

scholarly journals Geochemistry of Garga-Sarali intrusive granitoids (central domain of the central African fold belt in Cameroon): petrological implication

2020 ◽  
Vol 8 (1) ◽  
pp. 33
Author(s):  
Daama Isaac ◽  
Mbowou Gbambie Isaac Bertrand ◽  
Yamgouot Ngounouno Fadimatou ◽  
Ntoumbe Mama ◽  
Ngounouno Ismaïla
Keyword(s):  
Rare Earth ◽  
Rare Earth Elements ◽  
Partial Melting ◽  
Fractional Crystallization ◽  
Coarse Grained ◽  
Geochemical Analysis ◽  
Fold Belt ◽  
Volcanic Arcs ◽  
Fine Grained ◽  
Incompatible Elements

The Garga-Sarali granitoids outcrop in form of large slabs and undistorted large blocks, into a schisto-gneissic basement. These rocks contain mainly muscovite and microcline, followed by K-feldspar, quartz, biotite, pyroxene, zircon and oxides, with coarse-grained to fine-grained textures. Geochemical analysis show that it belongs to differentiated rocks group (granodiorite-granite) with high SiO2 (up to 72 wt%) contents. Their genesis was made from a process of partial melting and fractional crystallization. These rocks are classified as belonging to I- and S-Type, meta-peraluminous, shoshonitic granites; belonging to the domain of volcanic arcs. The rare earth elements patterns suggest a source enriched of incompatible elements. The Nb-Ta and Ti negative anomalies from the multi-element patterns are characteristics of the subduction domains.  

Zircon U–Pb and molybdenite Re–Os dating and geological implications of the Shimadong porphyry molybdenum deposit in eastern Yanbian, northeastern China

2020 ◽  
Vol 57 (5) ◽  
pp. 630-646
Author(s):  
Xi-Tao Nie ◽  
Jing-Gui Sun ◽  
Feng-Yue Sun ◽  
Bi-Le Li ◽  
Ya-Jing Zhang ◽  
...  
Keyword(s):  
Rare Earth ◽  
Rare Earth Elements ◽  
Lower Crust ◽  
Active Continental Margin ◽  
Major And Trace Elements ◽  
Northeastern China ◽  
Heavy Rare Earth ◽  
The North ◽  
Heavy Rare Earth Elements ◽  
Porphyry Mo Deposit

The Shimadong porphyry Mo deposit is located in eastern Yanbian, in the eastern part of the north margin of the North China craton, northeastern China. Here, we present the whole-rock major and trace elements, zircon U–Pb and Hf isotope data, and molybdenite Re–Os data for the Shimadong deposit. The porphyry was emplaced at 163.7 ± 0.9 Ma and the mineralization at 163.1 ± 0.9 Ma, suggesting that the mineralization was associated with the emplacement of the Shimadong porphyritic monzogranite. The porphyritic monzogranite had high SiO2 (70.09–70.55 wt%) and K2O + Na2O (7.98–8.27 wt%) contents and low MgO (0.51–0.53 wt%), TFeO (2.4–2.47 wt%), CaO (2.19–2.26 wt%), and K2O/Na2O (0.8–0.82) contents. The porphyry was rich in large ion lithophile elements Rb, Ba, K, and Sr, depleted in high-field-strength elements Y, Nb, Ta, P, and Ti, without significant Eu anomaly (δEu = 0.86–1.00), and depleted in heavy rare earth elements with light rare earth elements/heavy rare earth elements = 18.25–20.72 and (La/Yb)N = 27.10–34.67. These features are similar to those of adakitic rocks derived from a thickened lower crust. Zircon εHf(t) values for the porphyritic monzogranite ranged from –19.2 to 6.3, and the two-stage Hf model ages (TDM2) were 2421–811 Ma. These data indicate that the primary magma of the Shimadong porphyritic monzogranite was mainly derived from partial melting of the thickened lower crust consisting of juvenile crust and pre-existing crust. Combined with the results of previous studies, our data suggest that the Shimadong porphyry Mo deposit was emplaced along an active continental margin related to the westward subduction of the paleo-Pacific Plate.

Miocene high Sr/Y magmatism, south Tibet: Product of partial melting of subducted Indian continental crust and its tectonic implication

Lithos ◽  
2010 ◽  
Vol 114 (3-4) ◽  
pp. 293-306 ◽  
Author(s):  
Wang-Chun Xu ◽  
Hong-Fei Zhang ◽  
Liang Guo ◽  
Hong-Lin Yuan
Keyword(s):  
Partial Melting ◽  
Continental Crust ◽  
Tectonic Implication ◽  
South Tibet

Latest Cretaceous “A2-type” granites in the Sakarya Zone, NE Turkey: Partial melting of mafic lower crust in response to roll-back of Neo-Tethyan oceanic lithosphere

Lithos ◽  
2018 ◽  
Vol 302-303 ◽  
pp. 312-328 ◽  
Author(s):  
Orhan Karsli ◽  
Faruk Aydin ◽  
Ibrahim Uysal ◽  
Abdurrahman Dokuz ◽  
Mustafa Kumral ◽  
...  
Keyword(s):  
Partial Melting ◽  
Lower Crust ◽  
Oceanic Lithosphere ◽  
Sakarya Zone ◽  
Roll Back ◽  
Mafic Lower Crust

Two phases of post-onset collision adakitic magmatism in the southern Lhasa subterrane, Tibet, and their tectonic implications

2019 ◽  
Vol 132 (7-8) ◽  
pp. 1587-1602
Author(s):  
Tian-Yu Lu ◽  
Zhen-Yu He ◽  
Reiner Klemd
Keyword(s):  
Rare Earth ◽  
Lower Crust ◽  
Tectonic Setting ◽  
Geochemical Data ◽  
Magma Source ◽  
New Mineral ◽  
Element Abundances ◽  
Lhasa Terrane ◽  
Adakitic Rocks ◽  
Two Phases

Abstract Abundant Neogene adakitic magmatism occurred in the southern Lhasa subterrane after the onset of the India–Asia collision while convergence continued. However, the tectonic setting and magmatic evolution of the adakitic rocks are still under discussion. This study includes new mineral chemical and whole-rock geochemical data as well as zircon U-Pb and Lu-Hf isotopes of adakitic intrusive rocks from the Gyaca and Nyemo locations in the southern Lhasa subterrane. Laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) zircon U-Pb dating yielded crystallization ages of ca. 30 Ma for the Gyaca and Nyemo monzogranite and ca. 15 Ma for the Nyemo granodiorite. Both have common chemical signatures such as low MgO and heavy rare earth element contents as well as low compatible element abundances, indicating that these rocks result from partial melting of thickened lower crust with residual eclogite and garnet amphibolite. Furthermore, these rocks are characterized by variable positive zircon εHf(t) values, suggesting a juvenile magma source with variable ancient crustal contributions. Taking previous data into account, the adakitic magmatism concurs with an early late Eocene to Oligocene (ca. 38–25 Ma) and a late Miocene (ca. 20–10 Ma) phase. The adakitic rocks of the two phases are characterized by different fractionation evolutions of light and medium rare earth elements. We propose that the early-phase adakitic rocks were generated by the anatexis of Lhasa terrane lower crust owing to crustal shortening and thickening subsequent to the onset of the India–Asia collision and the upwelling of hot asthenosphere beneath the Lhasa terrane caused by the break-off of the Neo-Tethyan oceanic slab. The latest phase of adakitic rocks, however, relates to upwelling asthenosphere following the delamination and/or break-off of the subducting Indian continental slab.

S-type granite formation in the Dalradian rocks of Connemara, W. Ireland

1990 ◽  
Vol 54 (374) ◽  
pp. 1-22 ◽  
Author(s):  
Y. Ahmed-Said ◽  
B. E. Leake
Keyword(s):  
Rare Earth ◽  
Rare Earth Elements ◽  
Partial Melting ◽  
Fission Track ◽  
Sr Isotope ◽  
Crustal Rocks ◽  
Granite Formation ◽  
Type Granite ◽  
Gabbroic Intrusion ◽  
Detailed Picture

AbstractThe vicinity of the 490 Ma Cashel gabbroic intrusion experienced pressures of about 4.05 ± 0.2 kbar and temperatures in excess of 850 °C. These conditions caused intense hornfelsing and partial melting of the surrounding Dalradian metasediments. From the study of the progressively changed composition of the aureole hornfelses it is deduced that elements were fractionated into the melts as follows: Si>K>Na>Ca>Mn>Al>Fe>Mg and Rb>Ba>Sr>Ga>Cr,Ni,Co. This order of fractionation, which is the opposite to that in magmatic crystallization, provides a detailed picture of the mode of interaction between a mantle derived basic magma and mid-crustal rocks, illustrating how one type of S-type granite can be produced. The rare earth elements (REE) were both removed and fractionated but Eu largely remained in the crystal fractions giving increasing positive Eu anomalies with rising partial melting and these trends can be explained by the extraction of a granitic melt from the hornfelses. Fission track mapping of U is used to study the behaviour of U within the aureole and the metamorphic recrystallization of detrital brown zircon to pink new zircon. The S-type Cashel microgranite sill is shown to have been derived by anatexis from the Dalradian rocks, to have preserved the Sr isotope ratios of the metasediments at 490 Ma, and not to be of the same composition as the leucosomes in the metasediments.

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