青藏高原东西向伸展及其地质意义

东西和南北向伸展是青藏高原最显著的地质特征之一。南北向伸展形成的东西走向伸展构造,主要包括藏南拆离系(STDS),和沿喀喇昆仑—嘉黎断裂带(KJFZ)发育的正断层体系。东西向伸展形成数目众多的南北走向伸展构造,它们切割青藏高原几乎所有的东西走向构造单元,包括羌塘地块、KJFZ和STDS等,说明东西向伸展以整体形式发生并同时波及整个青藏高原,而不是由以KJFZ和STDS为边界的不同地块的不均匀挤出所致。南北走向伸展构造在地表呈之字形,为南北向挤压形成的追踪张断裂;剖面上表现为被后期高角度正断层叠加的拆离断层,拆离断层形成于中-晚中新世而高角度正断层形成于上新世及以后。导致拆离断层的东西向伸展可能是南北向挤压的变形分解,后期高角度正断层作用可能是高原隆升后的垮塌所致。东西向伸展是控制青藏高原新生代浅色花岗岩和盆地形成的主要因素。     

深熔过程中熔体成分与锆石行为模拟计算

发生深熔作用是高级变质作用的一个重要特征。深熔过程中产生的熔体可为淡色花岗岩提供潜在的源区;深熔过程中锆石的行为直接影响对变质锆石记年地质意义的理解。在含Zr体系下的相平衡模拟显示泥质成分深熔过程中产生熔体的成分在P-T空间中规律变化。温度升高时熔体Zr/Si值、Zr、FeO、MgO以及CaO等含量明显增加,压力较高时K2O含量也随温度升高而明显增加。Na2O含量随温度升高而降低,但随压力升高而增加。压力升高时Al/Si值显著升高。温度较高时Na/(Na+K)等值线较陡,减压熔融过程不会显著改变熔体Na/(Na+K)值,而升温减压过程以及近等压升温过程都会明显降低熔体Na/(Na+K)值。中压时随温度升高熔体Fe/(Fe+Mg)值缓慢升高,而石榴石的生长发育会迅速降低熔体Fe/(Fe+Mg)值。不同温压条件下对应的固相线熔融、白云母脱水熔融以及黑云母脱水熔融形成的熔体成分具有明显差异。对比模拟熔体成分在P-T空间的演化,喜马拉雅地区电气石淡色花岗岩对应熔体的形成温压条件应低于二云母淡色花岗岩,同类型淡色花岗岩之间在形成条件上也可能存在一定差异,并经历了差异性演化过程。含Zr体系下的相平衡关系显示进变过程是消耗锆石的过程,因而在进变过程中变质锆石难以生长,发生深熔作用的岩石中的变质锆石主要在退变过程中形成并记录退变质年龄。熔体丢失相关模拟显示不同温度阶段发生熔体丢失对锆石稳定性的影响不同。温度较低时Zr含量较少的熔体丢失会扩大持续进变过程中锆石的稳定范围,而温度较高时富Zr熔体的丢失会降低持续进变过程中锆石的稳定温度。类似于分离熔融作用的过程最利于残留相中剩余锆石在持续进变过程中的保存。     

Petrologic and geochemical links between the post-collisional Proterozoic Harney Peak leucogranite, South Dakota, USA, and its source rocks

The Proterozoic terrane of the Black Hills, South Dakota, includes the composite Harney Peak leucogranite and associated pegmatites that were emplaced into metamorphosed pelites and graywackes. Available dates indicate that granite generation post-dated regional metamorphism and deformation that have been attributed to collision of the Wyoming and Superior cratons at 1760 Ma. Previous radiogenic and stable isotope work indicates that the exposed metasedimentary rocks are equivalent to sources of the leucogranites. In this study, whole rock and mineral compositions of the metasedimentary rocks were used to calculate the likely average residue mineralogies and melt fractions that would be generated by muscovite dehydration melting of the rocks. These were then used to model observed trace element compositions of the granites using published mineral/melt distribution coefficients. Model trace element melt compositions using pelitic and graywacke protoliths yield similar results.

The models reproduce well the observed depletion of transition metals and Ba in the granites relative to metasedimentary protoliths. The depletion is due mainly to high proportion of biotite with variable amounts of K-feldspar in the model residue. Sr is also depleted in the granites compared to source rocks, but to a lesser relative extent than Ba. This is because of the low biotite/melt distribution coefficient for Sr and because high proportion of plagioclase in the residue is compensated by high Sr concentrations in protoliths. Rubidium, Cs and Ta behaved as slightly compatible to incompatible elements, and therefore, were not strongly fractionated during melting. Of the considered elements, only B appears to have been highly incompatible relative to residue during melting. The protoliths had sufficient B to allow tourmaline crystallization in those parts of the Harney Peak Granite in which Ti concentration was sufficiently low not to enhance crystallization of biotite.

The reproducibility of observed trace element concentrations in the Harney Peak Granite by the models supports the often made proposition that metapelites and metagraywackes are common sources for leucogranites. This argues against mass input from the mantle into metagraywacke and metapelitic crustal sources or melting of amphibolites to generate the post-collisional Harney Peak and other similar peraluminous granite suites.     

The Greer Lake leucogranite, Manitoba, and the origin of lepidolite-subtype granitic pegmatites

The Archean Greer Lake leucogranite intruded metabasalts of the Bird River Greenstone Belt in the southwestern part of the Superior Province of southeastern Manitoba. The considerably evolved, multiphase, peraluminous, B-, P-, and S-poor leucogranite (K/Rb 132 to 24) was probably generated by fault-friction-assisted anatexis of dominantly metatonalitic rocks and subsequent differentiation. The leucogranite produced interior, transitional, non-crosscutting pods of barren, beryl-columbite- and lepidolite-subtype pegmatites that solidified from local segregations of highly fractionated residual melt. Steep fractionation gradients characterize the granite-to-pegmatite transition, most conspicuously so in the case of the most evolved, Li, Rb, Cs, Be, Mn, Sn, Nb-Ta, F-rich, lepidolite-subtype pod AC #3 (with K/Rb ≥ 16 and Cs 330 ppmwt in accessory K-feldspar, ≥2.5 and ≤11,200 ppmwt, respectively, in lepidolite, Cs ≤28,000 ppmwt in beryl, and Ta/(Ta+Nb) at. ≤ 0.95 in manganotantalite). The Greer Lake example documents beyond any doubt the igneous derivation of lepidolite-subtype pegmatites from a plutonic parent. Most cases of generally very scarce lepidolite-subtype pegmatites obscure this relationship, as the volatile-rich, highly fluid melts stable to relatively low temperatures commonly migrate to great distances from their plutonic sources.     

高喜马拉雅淡色花岗岩形成的热模拟

地壳物质重熔形成高喜马拉雅淡色花岗岩需要在中地壳深度20多百万年前达到650℃以上的温度，但如何解释冷的印度板块向欧亚大陆俯冲时会产生这样的高温是一个难题，单一因素的解释往往不能解释全部地质观测事实．本文通过有限单元法定量化计算，提出多种因素的综合作用的假说，认为较合理的估计包括逆掩断层的幕式活动和间歇、摩擦剪切生热、高放射性元素含量和低热导率岩石的存在、高抬升剥蚀速度等因素．这些因素的组合才造成高喜马拉雅淡色花岗岩既能成带，但又只是断续的出现．     

Fractionated Ilmenite-series Granites in Southwest Japan: Source Magma for REE-Sn-W Mineralizations

Abstract. Various leucocratic biotite granites, low-temperature I-type, from the middle zone of the Sanyo ilmenite-series granitic terrane were studied chemically. These granites are locally associated with REE-Sn-W mineralizations, and were compared with unmineralized granites and batholithic Ryoke granites in three areas of the Chubu, Kinki and Chugoku Districts. They are unique in the region because they have extremely low ferromagnesian components but high Rb/Sr and 10000Ga/Al ratios. These granites are divided petrographically into the main phase, finer-grained marginal phase and younger sheets and dikelets. These rocks have increasing of HREE+Y and Nb+Ta contents in this order, which is also followed by decreasing zircon saturation temperature from 780 to 725C. Together with the mode of occurrence of these granites, the leucogranitic magmas are considered to have formed by in-situ fractionation of the host granitic magmas near the top of the magma chambers. The concentration of HREE, Y, Nb and Ta in these Sanyo Belt leucogranites is principally controlled by magmatic fractionation.     

Thermal Constraints on the Emplacement Rate of a Large Intrusive Complex: The Manaslu Leucogranite, Nepal Himalaya

The emplacement of the Manaslu leucogranite body (Nepal, Himalaya)has been modelled as the accretion of successive sills. Theleucogranite is characterized by isotopic heterogeneities suggestinglimited magma convection, and by a thin (<100 m) upper thermalaureole. These characteristics were used to constrain the maximummagma emplacement rate. Models were tested with sills injectedregularly over the whole duration of emplacement and with twoemplacement sequences separated by a repose period. Additionally,the hypothesis of a tectonic top contact, with unroofing limitingheat transfer during magma emplacement, was evaluated. In thislatter case, the upper limit for the emplacement rate was estimatedat 3·4 mm/year (or 1·5 Myr for 5 km of granite).Geological and thermobarometric data, however, argue againsta major role of fault activity in magma cooling during the leucograniteemplacement. The best model in agreement with available geochronologicaldata suggests an emplacement rate of 1 mm/year for a relativelyshallow level of emplacement (granite top at 10 km), uninterruptedby a long repose period. The thermal aureole temperature andthickness, and the isotopic heterogeneities within the leucogranite,can be explained by the accretion of 20–60 m thick sillsintruded every 20 000–60 000 years over a period of 5Myr. Under such conditions, the thermal effects of granite intrusionon the underlying rocks appear limited and cannot be invokedas a cause for the formation of migmatites. KEY WORDS: granite emplacement; heat transfer modelling; High Himalayan Leucogranite; Manaslu; thermal aureole     

藏南东部麻玛沟地区早古生代与中新世岩浆作用及其意义

喜马拉雅造山带东部错那县麻玛沟地区发育多种类型的花岗片麻岩和淡色花岗岩。锆石SHRIMP U-Pb地质年代学研究结果表明：花岗片麻岩（MM15）原岩结晶年龄为500.7±4.5Ma,含石榴子石淡色花岗岩中携带的继承性核部锆石年龄为498.6±3.4Ma,表明该地区经历了早古生代的岩浆作用事件。淡色花岗岩的结晶年龄区间为15.7~25.1Ma之间,为白云母脱水熔融的产物,可能是晚元古代-早古生代花岗质岩石发生低程度部分熔融的结果。锆石形态学表明该区的花岗片麻岩和淡色花岗岩均为过铝质花岗岩,并相对富集Cs、Rb、U、Pb,亏损Zr、Hf和低Nb/Ta比值,属于造山型花岗岩,支持该区域古生代岩浆作用事件与俯冲-碰撞造山作用相关,不是被动大陆边缘构造背景。结合前人数据推断：（1）从晚元古代末期开始,原特提斯洋向印度大陆的初始俯冲为自东向西的俯冲扩展模式;和（2）喜马拉雅造山带中新世淡色花岗岩为白云母脱水熔融和水致白云母熔融共同作用的结果,岩浆活动至少存在五个相。     

Behavior of apatite in granitic melts derived from partial melting of muscovite in metasedimentary sources

Fluid-absent and fluid-fluxed melting of muscovite in metasedimentary sources are two types of crustal anatexis to produce the Himalaya Cenozoic leucogranites. Apatite grains separated from melts derived from the two types of parting melting have different geochemical compositions. The leucogranites derived from fluid-fluxed melting have relict apatite grains and magmatic crystallized apatite grains, by contrast, there are only crystallized apatite grains in the leucogranites derived from fluid-absent melting. Moreover, apatite grains crystallized from fluid-fluxed melting of muscovite contain higher Sr, but lower Th and LREE than those from fluid-absent melting of muscovite, which could be controlled by the distribution of partitioning coefficient (D_Ap/Melt) between apatite and leucogranite. D_Ap/Melt in granites derived from fluid-absent melting is higher than those from fluid-fluxed melting. So, not only SiO₂ and A/CNK, but also types of crustal anatexis are sensitive to trace element partition coefficients for apatite. In addition, due to being not susceptible to alteration, apatite has a high potential to yield information about petrogenetic processes that are invisible at the whole-rock scale and thus is a useful tool as a petrogenetic indicator.©2021 China Geology Editorial Office.     

藏南定结地区早中新世淡色花岗岩的形成机制及其构造动力学意义

定结日玛那穹窿位于高喜马拉雅带中段,由花岗片麻岩、变泥质岩、变基性岩及大量淡色花岗岩等组成,经历了角闪岩-麻粒岩相变质作用。为厘定淡色花岗岩的形成机制以及与高级变质岩的关系,我们对淡色花岗岩和高级变质岩进行了全岩元素和Sr和Nd同位素组成和SHRIMP锆石U-Pb地质年代学测试。全岩元素和Sr-Nd同位素测试结果揭示淡色花岗岩具有以下特征:(1)高SiO₂ (>72%),高Al₂O₃ (>12%)和高A/CNK比值 (>1.0);(2)高Rb,低Sr,高Rb/Sr比值(>1.0);(3)高∑REE和明显的负Eu异常;(4)高Sr同位素初始比值(0.7621~0.8846)和低ε_Nd(t)值(-13.0~-20.2)。淡色花岗岩的高Rb/Sr比值和Sr-Nd同位素系统特征表明其形成机制为主要为白云母脱水部分熔融作用,源区为由花岗片麻岩和变泥质岩组成的混合源区。SHRIMP锆石U-Pb年代学研究揭示出定结地区淡色花岗岩具有21.0±0.7Ma和15.8±0.1Ma 2期年龄,花岗片麻岩的锆石变质增生边年龄为22.2±1.4Ma,与该区的榴辉岩退变质年龄一致。这些数据共同表明,花岗片麻岩和 变泥质岩在22~21Ma发生高级变质和深熔作用,形成早期淡色花岗岩岩浆,在~16Ma进一步深熔,形成晚期淡色花岗岩岩浆。