Comparison of Proterozoic and Phanerozoic rift-related basaltic-granitic magmatism

This paper compares the 1.67–1.47 Ga rapakivi granites of Finland and vicinity to the 1.70–1.68 Ga rapakivi granites of the Beijing area in China, the anorogenic 130 Ma granites of western Namibia, and the 20–15 Ma granites of the Colorado River extensional corridor in the Basin and Range Province of southern Nevada. In Finland and China, the tectonic setting was incipient, aborted rifting of Paleoproterozoic or Archean continental crust, in Namibia it was continental rifting and mantle plume activity that led to the opening of southern Atlantic at 130 Ma. The 20–15 Ma granites of southern Nevada were related to rifting that followed the Triassic–Paleogene subduction of the Farallon plate beneath the southwestern United States. In all cases, extension-related magmatism was bimodal and accompanied by swarms of diabase and rhyolite–quartz latite dikes. Rapakivi texture with plagioclase-mantled alkali feldspar megacrysts occurs in varying amounts in the granites, and the latest intrusive phases are commonly topaz-bearing granites or rhyolites that may host tin, tungsten, and beryllium mineralization. The granites are typically ferroan alkali-calcic metaluminous to slightly peraluminous rocks with A-type and within-plate geochemical and mineralogical characteristics. Isotope studies (Nd, Sr) suggest dominant crustal sources for the granites. The preferred genetic model is magmatic underplating involving dehydration melting of intermediate-felsic deep crust. Juvenile mafic magma was incorporated either via magma mingling and mixing, or by remelting of newly hybridized lower crust. In Namibia, partial melting of subcontinental lithospheric mantle was caused by the Tristan mantle plume, in the other cases the origin of the mantle magmatism is controversial. For the Fennoscandian suites, extensive long-time mantle upwelling associated with periodic, migrating melting of the subcontinental lithospheric mantle, governed by heat flow and deep crustal structures, is suggested.     

香港世界地质公园晚侏罗世荔枝庄组岩石组合及其成岩地质过程解析

中国东南大陆晚侏罗世地层普遍缺失,仅零星见于个别地区,香港新界东北的荔枝庄组即为其一。荔枝庄组出露于香港世界地质公园沉积岩园区的荔枝庄地区,自下往上由火山岩—沉积岩—火山岩组合而成,沉积岩中发育大型包卷层理和滑塌构造等典型沉积构造,是香港地区最具代表性的晚侏罗世火山—沉积岩系。通过实测地层剖面研究,确定其成岩过程大体上可划分为早期普林尼式火山爆发、中期破火山口湖相沉积和晚期普林尼式火山爆发三个阶段,以湖相沉积作用为主、火山喷发作用为辅;受晚期火山岩浆活动的影响,沉积岩层普遍发生硅化或炭化。荔枝庄组独特的岩石组合与形成的古地理环境,为探讨中国东南大陆中生代火山活动—沉积作用方式与成岩过程,提供了难得的研究实例。     

Sedimentary facies analysis and depositional model of gravity-flow deposits of the Yanchang Formation,southwestern Ordos Basin,NW China

During the deposition of the Chang-7 (Ch-7) and Chang-6 (Ch-6) units in the Upper Triassic, gravity flows were developed widely in a deep lake in the southwestern Ordos Basin, China. Based on cores, outcrops, well-logs and well-testing data, this paper documents the sedimentary characteristics of the gravity-flow deposits and constructs a depositional model. Gravity-flow deposits in the study area comprise seven lithofacies types, which are categorised into four groups: slides and slumps, debris-flow-dominated lithofacies, turbidity-current-dominated lithofacies, and deep-water mudstone-dominated lithofacies. The seven lithofacies form two sedimentary entities: sub-lacustrine fan and the slump olistolith, made up of three and two lithofacies associations, respectively. Lithofacies association 1 is a channel–levee complex with fining-/thinning-upward sequences whose main part is characterised by sandy debris flow-dominated, thick-bedded massive sandstones. Lithofacies association 2 represents distributary channelised lobes of sub-lacustrine fans, which can be further subdivided into distributary channel, channel lateral margin and inter-channel. Lithofacies association 3 is marked by non-channelised lobes of sub-lacustrine fans, including sheet-like turbidites and deep-lake mudstones. Lithofacies association 4 is represented by proximal lobes of slump olistolith, consisting of slides and slumps. Lithofacies association 5 is marked by distal lobes of slump olistolith, comprising tongue-shaped debris flow lobes and turbidite lobes. It is characterised by sandy debris flow, muddy debris flow-dominated sandstone and sandstone with classic Bouma sequences. Several factors caused the generation of gravity flows in the Ordos Basin, including sediment supply, terrain slope and external triggers, such as volcanisms, earthquakes and seasonal floods. The sediment supply of sub-lacustrine fan was most likely from seasonal floods with a high net-to-gross and incised channels. Triggered by volcanisms and earthquakes, the slump olistolith is deposited by the slumping and secondary transport of unconsolidated sediments in the delta front or prodelta with a low net-to-gross and no incised channels.     

改进后灰色神经网络及短期基坑沉降预测研究

在短期基坑沉降监测中,由于数据量少且呈非线性变化,沉降模型很难准确建立。灰色GM(1,1)对数据少、趋势性强、波动小的数据有较高的预测精度,但不能模拟复杂的非线性函数;BP神经网络可以对非线性数据进行学习训练,具有自学习、自适应能力;通过将GM(1,1)与BP神经网络组合,并优化网络部分的学习率、权值和阈值等,建立一种改进的灰色神经网络模型,该模型具有对非线性数据自学习、自适应能力和预测精度更高等优点。通过某基坑沉降监测分析,验证改进的灰色神经网络模型预测精度更高,适合短期建模,具有很好的实用性。     

三重加权变形监测预测模型及应用

针对GM(1,1)建模过程存在背景值、时间因素和初始条件3方面的不足,该文提出三重加权TPGM(1,1)预测模型。通过对背景值进行加权生成新的背景值,建立PGM(1,1)模型;在PGM(1,1)基础上考虑到时间因素,在求解灰参数时进行第2次加权建立DPGM(1,1)模型;最后考虑到初始条件对预测模型的影响,在DPGM(1,1)基础上进行第3次加权,建立TPGM(1,1)模型。通过实例分析,比较GM(1,1)、PGM(1,1)、DPGM(1,1)、TPGM(1,1)4种模型在变形监测数据处理中的拟合和预测结果,表明三重加权TPGM(1,1)模型拟合效果更好、预测精度更高;该模型具有前3种模型的优点,同时弥补了传统GM(1,1)存在的不足。     

Magma associations in Ediacaran granitoids of the Cachoeirinha‒Salgueiro and Alto Pajeú terranes,northeastern Brazil: Forty years of studies

Granitic magmatism in the Cachoeirinha‒Salgueiro and Alto Pajeú terranes in the Transversal Zone Domain of the Borborema Province, northeastern Brazil, occurred in three main time intervals: 650–620 Ma, 590–560 Ma and 545–520 Ma. The oldest one is characterized by intrusions of magmatic-epidote (mEp) bearing calc-alkalic (some with trondhjemitic affinities) and high-K calc-alkalic plutons, synkinematic to the main regional foliation, under contractional tectonic regime, and exhibits T_DM < 2.0 Ga and ƐNd (0.6 Ga) from −1 to −4, and δ¹⁸O (zircon) values from 7.1 to 10‰_VSMOW. O- and Nd-isotope data for the 650‒620 Ma group of plutons is compatible with partial fusion of subducted oceanic basaltic crust (mEp-bearing calc-alkalic tonalites/granodiorites, equivalent to adakites). Voluminous intrusions in the 590–560 Ma interval are represented by abundant mEp-free high-K calc-alkalic, peralkalic, ultrapotassic, mEp-bearing high-K calc-alkalic, and less abundant shoshonitic magmas. Nd-model ages for this group of plutons vary from 1.5 to 2.5 Ga and ƐNd (0.6 Ga) ranges from −8 to −20; δ¹⁸O (zircon) varies from 6.4 to 7.9‰_VSMOW. Values of δ¹⁸O (zircon) for the 590‒560 Ma old group of plutons coupled with Nd isotope data are compatible with remelting of crustal (negative ƐNd, 1.6 to 2.0 Ga old) source rocks. O- and Nd-isotope data for this group of plutons are compatible with underplating of basaltic magma in the base of the lower crust for the high-K calc-alkalic granitoids, coeval to transcurrent movements along sigmoidal shear zones. Intrusion of one shoshonitic (Serrote do Arapuá), one calc-alkalic (Riacho do Icó) plutons besides the peralkalic Manaíra-Princeza Isabel dike set have witnessed this transition from contractional to transcurrent movements along shear zones, around 610‒600 Ma.     

Observer presence may alter the behaviour of reef fishes associated with coral colonies

Although underwater visual census (UVC) is the most frequently used technique for quantifying reef fish assemblages, remote video analysis has been gaining attention as a potential alternative. In the South Atlantic Ocean, Millepora spp. (class Hydrozoa) are the only branching coral species; however, little is known about the ecological role that they play for reef fish communities. We compared these two observation methods (remote video and UVC) to estimate reef fish abundance and species richness associated with colonies of the fire‐coral Millepora alcicornis at Tamandaré Reefs, Northeast Brazil. Additionally, the two different techniques were used to compare species behaviour in association with fire‐corals in order to examine the biases associated with each technique and provide useful information for behavioural ecologists studying fish–coral associations. There were no differences in reef fish abundance or species richness sampled by remote video or UVC. However, a significant difference in the behaviour of associated fish was recorded between the two methods. In the presence of a diver carrying out a UVC, fish were observed spending more time sheltered amongst the coral branches compared with passively swimming on coral colonies with the remote video technique. Specifically, on the remote video recordings agonistic interactions between fish and passive swimming accounted for 33.3% and 22.2% of the census time, respectively. By comparison, when observed by a diver fish spent 34.8% of their time sheltering amongst the coral branches. We demonstrate that both techniques are similarly effective for recording fish abundance and species richness associated with fire‐corals. However, differences were observed in the ability of each method to detect the behaviour of coral‐associated fishes. Our findings show that behavioural ecologists studying complex fish–coral associations need to ensure that their aims are clearly defined and that they choose the most appropriate technique for their study in order to minimize methodological biases.     

天水盆地新近纪伸展构造——来自沉积与构造变形方面的证据

天水盆地位于青藏高原东北缘高海拔挤压隆升区与鄂尔多斯低海拔伸展区的过渡部位,新构造活动强烈。然而,新构造活动对天水盆地的影响尚不清楚。通过对盆地的沉积环境、构造沉降、构造变形等方面的研究,结果显示:1盆地由风成堆积、洪积扇、河湖相与湖泊相沉积组成,代表沉积中心的河湖相与湖泊相沿控盆断裂(西秦岭北缘断裂、西和断裂与礼县-罗家堡断裂)分布;2盆地经历了16~14Ma、9.2~7.4Ma和3.6~2.6Ma三次加速沉降期;3控盆断裂在同沉积期为正断层。沉积中心沿断裂分布、快速沉降事件及生长正断层表明,天水盆地至少在中新世晚期受控于走滑伸展构造,记录了青藏高原向北东方向的构造挤出作用。     

Isotope Geochronologic and Geochemical Constraints on the Magmatic Associations of the Collisional Orogenic Zone in the West Kunlun Orogen,China

The Jiajiwaxi pluton in the southern portion of the West Kunlun Range can be divided into two collision–related intrusive rock series, i.e., a gabbro–quartz diorite–granodiorite series that formed at 224±2.0 Ma and a monzonitic granite–syenogranite series that formed at 222±2.0 Ma. The systematic analysis of zircon U-Pb geochronology and bulk geochemistry is used to discuss the magmatic origin(material source and thermal source), tectonic setting, genesis and geotectonic implications of these rocks. The results of this analysis indicate that the parent magma of the first series, representing a transition from I-type to S-type granites, formed from thermally triggered partial melting of deep crustal components in an early island–arc–type igneous complex, similar to an I-type granite, during the continental collision orogenic stage. The parent magma of the second series, corresponding to an S-type granite, formed from the partial melting of forearc accretionary wedge sediments in a subduction zone in the late Palaeozoic–Triassic. During continued collision, the second series magma was emplaced into the first series pluton along a central fault zone in the original island arc region, forming an immiscible puncture-type complex. The deep tectonothermal events associated with the continent–continent collision during the orogenic cycle are constrained by the compositions and origins of the two series. The new information provided by this paper will aid in future research into the dynamic mechanisms affecting magmatic evolution in the West Kunlun orogenic belt.