Geochemistry of magmatic rocks and time constraints on deformational phases and shear zone slip in the Méiganga area,central Cameroon

The Méiganga area is situated in the Adamawa–Yadé domain (AYD) of the Pan-African fold belt in Cameroon. The AYD is characterized by abundant plutonic rocks that intruded Palaeoproterozoic gneisses. It is cut by the transcurrent Central Cameroonian Shear Zone (CCSZ). The studied area is made up of metadiorite (MD), pyroxene-bearing granite (PGr) and biotite-muscovite granite (BMGr), hosted in a metasedimentary sequence of amphibole-biotite gneisses. The granitoids are metaluminous to slightly peraluminous, and mainly of I-type (ASI ≤ 1.1), representing a high-K calc-alkaline to shoshonitic suite. They were derived from crustal protoliths (BMGr), from rocks of mantle origin (MD, PGr), and/or from the remelting of metabasalt or metatonalite (MD, PGr). Four successive deformational phases (D1, D2, D3, and D4) are present in the Méiganga area. The S1 foliation is formed by tectonic transposition of the S0 primitive surface represented by contacts between the gneiss and intercalated amphibolites. The synmigmatitic D2 deformational phase is characterized by S2 banded schistosity, S2/C2 sinistral shear planes, and F2 folds with axes parallel to a L2 stretching lineation. Imprints of the D2 and subsequent deformational phases are similar in the metadiorite and host rocks, implying syn-D2 emplacement and crystallization of the metadiorite; therefore 614–619 Ma ²⁰⁷Pb/²⁰⁶Pb zircon evaporation ages obtained for this rock type date the syn-D2 magmagenesis. Similarly, the D3 phase of the PGr is 601 ± 1 Ma, dated by the ²⁰⁷Pb/²⁰⁶Pb evaporation method. D4 is a late-stage brittle deformational phase. Sinistral movement of the CCSZ is associated with D2, whereas its latest activity, characterized by dextral slip, cannot be older than emplacement of the 558 ± 2 Ma BMGr (²⁰⁷Pb/²⁰⁶Pb zircon evaporation age).     

Sr,Nd and Pb isotopic and geochemical constraints for the origin of magmas in Popocatépetl volcano (central Mexico) and their relationship with the adjacent volcanic fields

We present new geochemical data (major- and trace-elements, as well as Sr and Nd isotopic compositions) of volcanic rocks erupted from Popocatépetl volcano during the volcanic event from December 2000 to January 2001. These data along with an exhaustive compilation of geochemical and Sr, Nd, and Pb isotope data reported for Popocatépetl rocks and nearby volcanic areas are used to examine the origin and geochemical evolution of the magmas in the central Mexican volcanic belt (CMVB). During this period of volcanic eruptions Popocatépetl produced ash columns as high as 7 km. Pyroclastic flows and lahars were observed after the completion of the activity. Samples of banded pumice and a bomb fragment transported by the lahar were chemically analysed for this work. Rocks show an andesitic composition with 58.5–61.7 wt.% SiO₂ and 5.9–4.0 wt.% MgO. Contents of large ion lithophile elements (LILE), rare-earth elements (REE) and Zr are nearly constant through the compositional range. No significant Eu anomaly is present, but the samples show Nb-anomaly relative to LILE and high-field strength elements (HFSE). Nd- and Sr-isotopic compositions of these samples range from ¹⁴³Nd/¹⁴⁴Nd = 0.51291 to 0.51287 and ⁸⁷Sr/⁸⁶Sr = 0.70399 to 0.70422. Comparison of Popocatépetl products with volcanic rocks from the nearby areas shows that the magmas in CMVB were generated in a heterogeneously veined-mantle source enriched in LILE, HFSE, and REE. Additional crustal assimilation as well as fractional crystallization could account for the great chemical variability of rocks in the CMVB. Statistical comparison of the geochemical compositions of the volcanic products ejected from 1994 to 2000 to those ejected during the 2001 event shows that most geochemical parameters (major- and trace-elements, normative minerals, Sr and Nd isotopic composition, as well some elemental ratios) present no statistically significant differences. Statistically significant differences in the mean only were computed for the major-elements SiO₂, FeO, MgO, CaO, and K₂O, as well as for the rare-earth elements Nd, Sm, Eu, Gd, Dy, Ho, Tm, and Yb.     

Re–Os and U–Pb geochronology of porphyry Mo deposits in central Jilin Province: Mo ore-forming stages in northeast China

Central Jilin Province lies along the eastern edge of the Xing–Meng orogenic belt of northeast China. At least 10 Mo deposits have been discovered in this area, making it the second-richest concentration of Mo resources in China. To better understand the formation and distribution of porphyry Mo deposits in the area, we investigated the geological characteristics of the deposits and applied zircon U–Pb and molybdenite Re–Os isotope dating to constrain the age of mineralization. Our new geochronological data show the following: the Jidetun Mo deposit yields molybdenite Re–Os model ages of 164.6–167.1 Ma, an isochron age of 168 ± 2.5 Ma, and a weighted mean model age of 165.9 ± 1.2 Ma; the Houdaomu Mo deposit yields molybdenite Re–Os model ages of 167.4–167.7 Ma, an isochron age of 168 ± 13 Ma, and a weighted mean model age of 167.5 ± 1.2 Ma; and the Chang’anpu Mo deposit yields a zircon U–Pb age for granodiorite porphyry of 166.9 ± 1.5 Ma (N = 16). These new age data, combined with existing molybdenite Re–Os dates, show that intense porphyry Mo mineralization was coeval with magmatism during the Middle Jurassic (167.8 ± 0.4 Ma, r > 0.999). The geotectonic mechanisms responsible for Mo mineralization were probably related to subduction of the Palaeo-Pacific plate beneath the Eurasian continent. Combining published molybdenite Re–Os and zircon U–Pb ages for northeast China, the Mo deposits are shown to have been formed during multiple events coinciding with periods of magmatic activity. We identified three phases of mineralization, two of which had several stages: the Caledonian (485–480 Ma); the Indosinian comprising the Early–Middle Triassic (248–236 Ma) and Late Triassic (226–208 Ma) stages; and the Yanshanian phase comprising the Early–Middle Jurassic (202–165 Ma), Late Jurassic–early Early Cretaceous (154–129 Ma), and Early Cretaceous (114–111 Ma) stages. Although Mo deposits formed during each phase/stage, most of the mineralization occurred during the Early–Middle Jurassic.     

U-Pb zircon and K-Ar geochronology reveal the emplacement and cooling history of the Late Cretaceous Beypazarı granitoid,central Anatolia,Turkey

The Beypazar? granitoid has been studied with respect to multi-radiometric dating and oxygen isotopic geothermometry. Radiometric dating of the granitoid yields zircon U-Pb isochron ages ranging from 72.5 ± 12.6 to 78.6 ± 4.7, and K-Ar ages of 71.4 ± 2.8 to 74.9 ± 2.9 and 59.5 ± 2.2 to 75.4 ± 2.9 Ma for hornblende and biotite, respectively. Oxygen isotope thermometry for the granitoid gives temperatures of 550 ± 25°C to 605 ± 30, 390 ± 15 to 540 ± 25°C, and 481 ± 5 to 675 ± 10°C, for hornblende, biotite, and K-feldspar, respectively, when paired with quartz. The systematic differences among ages according to different techniques used on different minerals are used to reconstruct the cooling history of the granite. The results yield rapid cooling rates of 33.3°C/Ma from 800°C to 550°C, and slow cooling rates of about 15 ± 0.5°C/Ma from 550 to 300°C. Rapid subsolidus cooling between 600°C and 550°C is documented by ⁴⁰Ar/³⁹Ar ages on amphibole and biotite between 71.4 ± 2.8 and 75.4 ± 2.9 Ma. Younger ages on biotites from two samples (59.5 ± 2.2 and 64.4 ± 2.5) are probably caused by loss of Ar. The reason for this possible Ar loss can be interpreted as slower subsolidus cooling (~375°C) ages. There is an apparent spatial and temporal relationship between the intrusion-cooling of the Beypazar? granitoid and the evolution of the ?zmir–Ankara–Erzincan ocean belonging to the northern Neo-Tethyan ocean domain.     

Petrology,geochemistry, and geochronology of mafic rocks from the Taoxinghu Devonian ophiolite,LongmuCo–Shuanghu–Lancang suture zone,northern Tibet: evidence for an intra-oceanic arc–basin system

Abstract

A newly discovered Devonian ophiolite located in the Taoxinghu area of central Qiangtang on the Qinhai–Tibet Plateau is described. The ophiolite consists of gabbro and diabasic dikes, and invasive cumulate gabbros-leucogabbros. The ophiolite has undergone greenschist facies metamorphism and minor deformation. Dating of the metagabbro by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U–Pb techniques yielded a weighted mean age of 367.2 ± 3.3 Ma (Late Devonian). Whole-rock geochemical analyses show that the rocks belong to the tholeiite series, with weak depletion in light rare-earth elements (LREEs), almost no Eu anomalies, weak enrichment in large-ion lithophile elements (LILEs), depletion in Nb and Ta, and weak negative Hf and Ti anomalies. These characteristics are similar to those of back-arc basin basalts. Together, these characteristics suggest that the rocks of the Devonian ophiolite formed by ~30% partial melting of spinel lherzolite, which was enriched by interaction with aqueous fluids during the late-generation phases; there is no evidence of subduction-related melting. The Devonian ophiolite rocks in the Taoxinghu area were first discovered in the LongmuCo–Shuanghu–Lancang suture zone. Detailed geochemical analyses show that the rocks formed in a back-arc ocean basin environment, indicating that the LongmuCo–Shuanghu–Lancang suture zone in central Qiangtang represents a Late Devonian intra-oceanic subduction zone in the Palaeo–Tethys Ocean. The discovery of the central Qiangtang Devonian ophiolite provides essential data for understanding the evolutionary history of the Palaeo–Tethys Ocean, and for identifying and understanding the roles of the different tectonic units on the Qinghai–Tibet Plateau.     

Petrogenesis of Early Cretaceous volcanic rocks of the Manketouebo Formation in the Wuchagou region,central Great Xing’an Range,NE China,and tectonic implications: geochronological,geochemical, and Hf isotopic evidence

ABSTRACT

This study presents new whole-rock major and trace element geochemistry, zircon U–Pb ages, and Hf-isotope compositions for volcanic rocks from the Manketouebo Formation of the central Great Xing’an Range, NE China. These data provide precise ages and information on the petrogenesis and source of the magmas that formed this formation, furthering our understanding of the geodynamic setting of the large-scale late Mesozoic magmatism in the Great Xing’an Range and other areas in NE China. The Manketouebo Formation in the study area is dominated by rhyolites and rhyolitic tuffs with minor trachydacites. The LA-ICP-MS zircon U–Pb dating indicates that these volcanic rocks formed between 143 and 139 Ma. The volcanic rocks contain high silica (66.70–79.91 wt.%) and total alkali (5.93–9.72 wt.%) concentrations, and low concentrations of MgO (0.08–1.15 wt.%), total FeO (0.68–4.50 wt.%), and CaO (0.10–2.56 wt.%). They are enriched in large-ion lithophile elements (LILEs; e.g. Rb, Th, and U) and light rare earth elements (LREEs), and depleted in high field strength elements (HFSEs; e.g. Nb, Ta, Ti, and P) and heavy rare earth elements (HREEs), indicating that they are similar to highly fractionated I-type igneous rocks. All of the magmatic zircons from the analysed samples have high initial ¹⁷⁶Hf/¹⁷⁷Hf ratios (0.282900–0.283093), positive ε_Hf(t) values (7.48–14.19), and young Hf two-stage model ages (954–344 Ma) that suggest the primary magma that formed the volcanic rocks of the Manketouebo Formation was derived from the partial melting of Neoproterozoic to Phanerozoic juvenile crustal material, indicating in turn that significant crustal growth occurred at this time within the Xing’an Terrane. These data, combined with previous research into the spatial–temporal distribution of Mesozoic volcanic rocks in NE China, suggest that the Early Cretaceous magmatism in the Great Xing’an Range was influenced by both the subduction of the Palaeo-Pacific Plate and the closure of the Mongol–Okhotsk Ocean. This was a crucial period in the transformation from the Mongol–Okhotsk Ocean to the Palaeo-Pacific tectonic regimes. In summary, the early stages of Early Cretaceous magmatism in this area were related to the closure of the Mongol–Okhotsk Ocean, whereas the later stages of magmatism in this area and elsewhere in NE China were related to the subduction of the Palaeo-Pacific Plate.     

内蒙古阿尔山绿水碱长花岗岩锆石U——Pb 年龄、地球化学特征及构造意义

绿水碱长花岗岩锆石的LA--ICP--MS U--Pb 同位素测试结果为139. 4 ± 1. 6 Ma,表明其形成于早白垩世。地球化学特征显示绿水碱长花岗岩属于弱过铝质高钾钙碱性A2 型花岗岩,具高硅、高碱和低CaO、Fe2O3、MgO 和P2O5 特征; 轻稀土元素( LＲEE) 富集,重稀土元素( HＲEE) 相对亏损,铕具有明显的负异常; 高场强元素Zr、Hf 和大离子亲石元素Ｒb、Th、U 及K 相对富集,Ba、Nb、Ta、 Sr 和P 具明显的亏损。结合区域地质资料,认为这些碱长花岗岩的形成可能与古太平洋板块俯冲导致的加厚岩石圈拆沉后伸展环境有关。     

福建省大地构造单元划分及基本特征

以地层划分、对比、沉积岩建造、火山岩建造、侵入岩浆活动和变质变形等地质记录为基础,以板块理论为指导,对福建省大地构造单元进行了重新认识和划分。将福建省区划分为华夏地块( Ⅴ --3) 、东南沿海岩浆弧( Ⅴ--4) 和闽中结合带( Ⅴ--7) 等3 个Ⅱ级构造单元,武夷古弧盆系( Ⅴ--3-- 1) 、南平-宁化( 夭折) 裂谷( Ⅴ--3--2) 、闽西南陆表海盆地( Ⅴ--3--3) 、闽东沿海岩浆弧( Ⅴ--4--1) 和松溪-尤溪蛇绿混杂岩( Ⅴ--7--1) 等5 个Ⅲ级单元,以及建宁古弧后盆地( Ⅴ--3--1--1) 等18 个Ⅳ 级构造单元。     

滇中易门县狮子山铜矿及近外围成矿因素及找矿方向

滇中易门县狮子山铜矿是产于扬子古陆边缘昆阳裂谷易门裂陷盆地内典型东川式层状矿床, 是在昆阳裂谷背景下经同生沉积和二次富集作用而成, 在鹅头厂组、落雪组、因民组等地层中均有不同矿体产出,矿床成因属热卤水沉积型东川式铜矿床。矿床的形成经历了同生阶段、成岩阶段和后生阶段。矿床在同生沉积阶段形成, 成岩阶段固结成岩, 后期的岩浆活动对矿床起到一定富集和改造作用。本文研究认为, 矿床受地层与岩性、岩相古地理、构造、岩浆岩等因素的控制, 通过对矿床成矿要素的分析和总结, 归纳了该区的找矿标志, 提出了矿区本部和近外围的找矿方向和靶区优选。     

Palaeozoic synorogenic sedimentation in central and northern Australia: A review of distribution and timing with implications for the evolution of intracontinental orogens

The Palaeozoic Alice Springs Orogeny was a major intraplate tectonic event in central and northern Australia. The sedimentological, structural and isotopic effects of the Alice Springs Orogeny have been well documented in the northern Amadeus Basin and adjacent exhumed Arunta Inlier, although the full regional extent of the event, as well as lateral variations in timing and intensity are less well known. Because of the lack of regional isotopic data, we take a sedimentological approach towards constraining these parameters, compiling the location and age constraints of inferred synorogenic sedimentation across a number of central and northern Australian basins. Such deposits are recorded from the Amadeus, Ngalia, Georgina, Wiso, eastern Officer and, possibly, Warburton Basins. Deposits are commonly located adjacent to areas of significant basement uplift related to north‐south shortening. In addition, similar aged orogenic deposits occur in association with strike‐slip tectonism in the Ord and southern Bonaparte Basins of northwest Australia. From a combination of sedimentological and isotopic evidence it appears that localised convergent deformation started in the Late Ordovician in the eastern Arunta Inlier and adjacent Amadeus Basin. Synorogenic style sedimentation becomes synchronously widespread in the late Early Devonian and in most areas the record terminates abruptly close to the end of the Devonian. A notable exception is the Ngalia Basin in which such sedimentation continued until the mid‐Carboniferous. In the Ord and Bonaparte Basins there is evidence of two discrete pulses of transcurrent activity in the Late Devonian and Carboniferous. The sedimentological story contrasts with the isotopic record from the southern Arunta Inlier, which has generally been interpreted in terms of continuous convergent orogenic activity spanning most of the Devonian and Carboniferous, with a suggestion that rates of deformation increased in the mid‐Carboniferous. Either Carboniferous sediments have been stripped off by subsequent erosion, or sedimentation outpaced accommodation space and detritus was transported elsewhere.