Behaviour of boron in the generation of an anatectic complex: The Pen˜a Negra complex, central Spain

The Pe˜na Negra anatectic complex (central Spain) was formed from a pelitic protolith low in boron (3–80 ppm), by dehydration reactions, melting and fluid loss, which expelled much of the boron; residual B is present in sillimanite and muscovite. Migmatite melanosomes and restites (sillimanite + biotite + ilmenite) are relatively richer in B (mean 29 ppm), while the leucosomes are poorer (13 ppm). Late shears, fractures and zones of retrograde alteration have permitted some reintroduction of B. Experiments demonstrated that B cannot be removed from these rocks by heating or by leaching, indicating that very little is present in superficial adsorption and that the element occurs in mineral structures.     

河北平山晚太古代阜平群中主要变质岩类及其深熔产物的稀土地球化学初步研究

河北平山小觉地区晚太古代阜平群含有三种主要表壳岩性质的变质岩类，即钾长浅粒岩类，黑云变粒岩－片麻岩类和角闪质岩石类。它们所经历的初期深熔作用产物，其岩石化学成分分别与钾长花岗岩，奥长花岗岩和花岗闪长岩相当，并在很大程度上继承了原岩（母岩）的总体岩石化学特征，可能由于深熔产物中富含稀土的一些副矿物含量的降低，它们的稀土总量都不同程度地较原岩为低，但两的稀土分布模式却很相似，这从另一角度说明了它们的成因关系。     

Post-collisional strongly peraluminous granites

Strongly peraluminous (SP) granites have formed as a result of post-collisional processes in various orogens. In ‘high-pressure' collisions such as the European Alps and Himalayas, post-collisional exhumation of overthickened crust (>50 km), heated by radiogenic decay of K, U and Th during syn-collisional thickening, produced small- to moderate-volume, cool (<875°C) SP granite melts with high Al₂O₃/TiO₂ ratios. In ‘high-temperature' collisions such as the Hercynides and Lachlan Fold Belt (LFB), there was less syn-collisional crustal thickening (≤50 km). Crustal anatexis was related to post-collisional lithospheric delamination and upwelling of hot asthenosphere, forming large-volume, hot (≥875°C) SP granite melts with low Al₂O₃/TiO₂ ratios. Both clay-rich, plagioclase-poor (<5%) pelitic rocks and clay-poor, plagioclase-rich (>25%) psammitic rocks have been partially melted in high-pressure and high-temperature collisional orogens, with the pelite-derived SP granites tending to have lower CaO/Na₂O ratios (<0.3) than their psammite-derived counterparts. The predominance of pelite-derived SP granites in the Himalayas and psammite-derived SP granites in the LFB suggests that mature continental platforms made up more of the accreted crust in the Himalayan collision than in the LFB.     

Garnet fractionation,progressive melt loss and bulk composition variations in anatectic metabasites:Complications for interpreting the geodynamic significance of TTGs

Tonalite-trondhjemite-granodiorite(TTG) suites constitute a large proportion of the Archean geological record;however,the geodynamic processes that generated them,and Archean continental crust in general,remain a subject of debate.The concentrations and ratios of Sr,Y,La,Yb,Nb,and Ta in TTGs are commonly used to determine the depth of melting of their metabasic sources.The trace element composition of melt produced by metabasic source rocks during anatexis is strongly affected by the presence and abundance of pressure-sensitive minerals,such as plagioclase(Sr-bearing),garnet(Y-and HREE-bearing),and rutile(Nb-and Ta-bearing).Elevated Sr/Y and La/Yb ratios and low concentrations of Nb and Ta in TTGs are generally considered to indicate melting at high pressures(≥2.0 GPa).The depth of melting is a key factor in determining the origin of TTGs as this provides critical information on the tectonic setting of their generation.We use phase equilibrium and trace element modelling to explore the effects of three potential influences on TTG trace element compositions:fractionation of trace elements into peritectic garnet cores,progressive melt loss from the source,and source bulk composition.We model three different compositions of Archean basalts along thermal gradients of 500℃/GPa,750℃/GPa,and 1000℃/GPa.The models produce maj or and trace element melt compositions that are generally consistent with measured compositions of TTGs.Although Sr/Y,La/Yb,Nb,and Ta exhibit pressure-dependent behaviour,other factors also affect these values.Garnet fractionation causes Sr/Y and La/Yb to reach much greater values and in this scenario,the values also increase with increasing temperature.Source bulk composition has an effect in all scenarios and most strongly influences La/Yb,Nb,and Ta.Overall,these results show that Sr/Y,La/Yb,Nb,and Ta can reach values generally considered to be indicative of high pressure melting at a range of P-T conditions including P 2.0 GPa.Consequently,trace element compositions of TTGs alone may provide a misleading impression of the depth of melting of metabasites and the geodynamic environment of Archean crustal growth and reworking.     

长英质高级片麻岩中夕线石的形成与变形-变质-深熔作用的关系 —— 以南极拉斯曼丘陵区为例

南极拉斯曼丘陵高级长英质片麻岩的夕线片麻岩中可有两类结构和变质矿物组合均有所不同的两种域,一种含夕线石部分对应于片理组合,另一种对应无夕线石的非片理化组合.岩石的变形尤其是破裂性裂隙的率先出现对于富夕线石部分的形成是必要的.在非破裂性片麻理岩石域中,中-低压/高温条件下黑云斜长片麻岩进变质发展的结果往往是形成Grt+Qtz±Opx组合.这两种不同的变质域的组合与应变分解造成的强应变带和弱应变域相一致.而且,夕线石的形成不是简单的变质早期矿物固相反应的结果,而是反应链上的一部分.其出现是由开放体系中组分的差异迁移造成的,这种差异迁移实际上是碱土金属迁出(淋滤)的过程,与变形相伴的流体活动使得SiO2发生强烈淋滤,残留组分中SiO2活度大为降低,并使长英质组分和镁铁质组分分凝,主要组分大都可以单独富集(集中)、形成复杂的矿物演化和分布.这种演化还可从MgO等碱(土)金属组分的外迁程度差异来理解.随着碱(土)金属丢失程度的减小,依次出现夕线石、石榴子石、斜方辉石和堇青石,或者说,不同的变质或分异阶段形成不同的矿物(组合):变形-变质起始阶段,碱(土)金属组分迁移初期残留形成夕线石,之后为镁(铁)质组分迁移,初期残留不透明钛铁氧化物,晚期残留组分形成堇青石.石榴子石-长英质组合为体系基本封闭情况下的结晶.此外,夕线石的形成往往标志着深熔作用的开始,一旦深熔作用发展完善,夕线石呈准稳定状态或趋于消失.拉斯曼丘陵与夕线石有关的长英质岩石经历了复杂的变形、变质和流体活动变化.     

桐柏造山带深熔作用:混合岩LA-ICPMS锆石U-Pb年代学证据

桐柏造山带是研究秦岭-桐柏-大别-苏鲁变质带演化的关键地区。由于桐柏高级变质杂岩深熔作用发生时间还缺乏准确的限定,这一区域的构造演化仍存在较大的争议。本文对桐柏杂岩中的一个混合岩的中色体和两个混合岩浅色体样品中的锆石进行了LA-ICPMS年代学测定。中色体中锆石分析点获得的上下交点分别为859±73Ma和135±250Ma。接近上交点的6个谐和分析点给出的²⁰⁶Pb/²³⁸U加权平均结果为828±7Ma (MSWD=0.57)。这一年龄结果同上交点在误差范围内一致,代表混合岩原岩结晶年龄,对应扬子板块北缘出现的中-新元古代的岩浆事件。另一方面,混合岩浅色体中的新生锆石具有面状分带或是弱的振荡环带,低的Th/U比值,锆石形态学和内部结构也表明新生锆石结晶于与深熔作用有关的熔体中,它们的²⁰⁶Pb/²³⁸U加权平均值分别为135±4Ma和131±3Ma。这一年龄范围代表桐柏高级变质地体发生深熔作用时间,区域上与桐柏-大别变质带广泛出现的碰撞后岩浆事件的时代相同。桐柏造山带出现造山后伸展的时间应不晚于135Ma。     

马拉山穹窿的活动时限及其在藏南拆离系-北喜马拉雅片麻岩穹窿形成机制的应用

本文报道吉隆北喜马拉雅地区马拉山穹窿核部浅色花岗岩的锆石SHRIMP U-Pb和白云母激光40Ar/39 Ar年代学研究.花岗岩U-Pb年龄显示,穹窿核部浅色花岗岩岩浆活动(深熔及侵位)发生于～30Ma至～17Ma,其中最年轻的U-Pb年龄(17Ma)以及花岗岩白云母40Ar/39Ar年龄(17～15Ma)指示了马拉山穹窿的最后岩浆侵位时间及可能的穹窿冷却事件.已有研究表明,北喜马拉雅片麻岩穹窿带(NHGD)与藏南拆离系(STDS)中浅色花岗岩具有相似的最早侵位年龄,即～35 Ma,而STDS下盘U-Pb年龄老于35Ma的浅色花岗岩为增厚地壳重熔成因,表明北喜马拉雅在 ～35Ma地壳构造体制由挤压转为伸展,并暗示在始新世-渐新世转换期可能存在一更广泛意义的地质事件.～35 Ma以前增厚导致中下地壳部分熔融,形成中下地壳渠道流,渠道流活动触发增厚造山楔的垮塌,形成STDS.STDS的伸展减薄引发更大规模浅色花岗岩侵位,花岗岩底辟作用形成了NHGD,本文最年轻U-Pb年龄及40Ar/39Ar年龄(17～15Ma)即代表马拉山的底辟与穹窿作用,之后的构造体制由东西向伸展所取代(始于～13Ma).     

8411地区深熔作用的Sr,Nd同位素地质研究

8411地区深熔作用的同位素地质研究表明,J_(1-2)砂岩是以元古代古陆壳供源的湖相沉积岩、岩浆岩为元古代古陆在下地壳局部重熔或重熔-混染的产物。铀的预富集与深熔作用中硅铝质地壳岩浆混染有关。铀矿物和岩浆岩的同位素组成相似。这些研究结果,结合岩石学、矿物学、同位素演化和稀土含量分析,认识到铀来自深源,元古代地层是铀源层。     

花岗质岩石中微粒交生体的成因研究

在一些花岗质片麻岩中,经常看到分布在长石颗粒周围的细小矿物集合体——微粒交生体。电子探针分析结果表明,微粒交生体主要由钠长石、钾长石和石英组成,其标准矿物平均成分相当于An-Ab-Or体系的“低限熔点”成分。显然,微粒交生体是花岗质片麻岩经部分熔融产生的“低限熔体”的结晶产物。太古宙高级区一些晚构造钾质花岗岩的矿物,化学特征与微粒交生体类似。这意味着在塑性变形过程中产生和聚集的“低限熔体”最终可以形成大规模钾质花岗岩。     

Neoproterozoic ana texts of the yunkai metamorphic Complex; Evidence from U-Pb geochronology of zircon in migniatite北大核心CSCD

In order to determine the migmatization time of the Preeambrian metamorphic basement(the Yunkai metamorphic complex) of the Yunkai terrain, a study of LA-MC-1CP-MS U-Pb dating of zircon in matrixes and felsic veins of mig-matites from the Tiantangshan Group. Results show that the inherited magmatic cores of zircon grains from the matrix of migmatite have oscillatory zoning and yield a U-Pb age of 1086 ±24 Ma, representing the pmtolith age, whereas the metamprohic rims of zircon grains from the felsic vein of migmatite yield an age of 852. 6±8. 8 Ma, representing the timing of the anatex is (migmatization). These results imply that there is a Mesoproterozoic metamorphic basement with a Neoproterozoic anatexis event in the Yunkai area. The timing of this anatexis event is consistent with that of the Sibao Movement. Therefore, we conclude that the timing of the anataxis could be an important geological record for the amalgamation of the Yangtze and Cathaysian blocks.