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1.
We report the results of a SIMS U-Pb study of 112 zircons from breccia samples from the Apollo 14 and 17 landing sites. Zircon occurs in the breccia matrices as rounded, irregular shaped, broken and rarely euhedral grains and as constituent minerals in a variety of lithic clasts ranging in composition from ultra-mafic and mafic rocks to highly evolved granophyres. Crystallisation of zircon in magmatic rocks is governed by the zirconium saturation in the melt. As a consequence, the presence of zircon in mafic rocks on the Moon implies enrichment of their parent melts in the KREEP component. Our SIMS results show that the ages of zircons from mafic to ultramafic clasts range from ca. 4.35 Ga to ca. 4.00 Ga demonstrating multiple generations of KREEPy mafic and ultramafic magmas over this time period. Individual zircon clasts in breccia matrices have a similar age range to zircons in igneous clasts and all represent zircons that have been incorporated into the breccia from older parents. The age distributions of zircons from breccias from both the Apollo 14 and Apollo 17 landing sites are essentially identical in the range 4.35-4.20 Ga. However, whereas Apollo 14 zircons additionally show ages from 4.20 to 3.90 Ga, no zircons from Apollo 17 samples have primary ages less than ca. 4.20 Ga. Also, in contrast to previous suggestions that the magmatism in the lunar crust is continuous our results show that the zircon age distribution is uneven, with distinct peaks of magmatic activity at ca. 4.35 Ga, ca. 4.20 Ga in Apollo 14 and 17 and a possible third peak in zircons from Apollo 14 at ca. 4.00 Ga. To explain the differences in the zircon age distributions between the Apollo 14 and 17 landing sites we propose that episodes of KREEP magmatism were generated from a primary reservoir, and that this reservoir contracted over time towards the centre of Procellarum KREEP terrane. We attribute the peaks in KREEP magmatism to impact induced emplacement of KREEP magma from a primary mantle source or to a progressive thermal build-up in the mantle source until the temperature exceeds the threshold for generation of KREEP magma, which is transported into the crust by an unspecified possibly plume-like process.  相似文献   

2.
Northwest Africa (NWA) 4472 is a polymict lunar regolith meteorite. The sample is KREEP-rich (high concentrations of potassium, rare earth elements and phosphorus) and comprises a heterogeneous array of lithic and mineral fragments. These clasts and mineral fragments were sourced from a range of lunar rock types including the lunar High Magnesian Suite, the High Alkali Suite, KREEP basalts, mare basalts and a variety of impact crater environments. The KREEP-rich nature of NWA 4472 indicates that the sample was ejected from regolith on the nearside of the Moon in the Procellarum KREEP Terrane and we have used Lunar Prospector gamma-ray remote sensing data to show that the meteorite is most similar to (and most likely sourced from) regoliths adjacent to the Imbrium impact basin.U-Pb and Pb-Pb age dates of NWA 4472 phosphate phases reveal that the breccia has sampled Pre-Nectarian (4.35 Ga) rocks related to early episodes of KREEP driven magmatism. Some younger phosphate U-Pb and Pb-Pb age dates are likely indicative of impact resetting events at 3.9-4 Ga, consistent with the suggested timing of basin formation on the Moon. Our study also shows that NWA 4472 has sampled impact melts and glass with an alkali-depleted, incompatible trace element-rich (high Sc, low Rb/Th ratios, low K) compositional signature not related to typical Apollo high-K KREEP, or that sampled by KREEPy lunar meteorite Sayh al Uhaymir (SaU) 169. This provides evidence that there are numerous sources of KREEP-rich protoliths on the Moon.  相似文献   

3.
Dhofar 025 is a lunar highland breccia consisting mainly of anorthositic, with less common noritic, gabbronoritic, and troctolitic material. Rare fragments of low-Ti basalts are present as well, but no KREEP (component enriched in incompatible elements) was found in the meteorite. The cathodoluminescence study of this meteorite showed that its impact–melt matrix contains unusual cathodoluminescent (CL) objects of feldspathic composition, which frequently contain microlites of Fe-Mg spinel (pleonaste). They were presumably formed by impact mixing and melting of olivine and plagioclase with subsequent rapid quenching of the impact melts. Such mixing could happen either during assimilation of anorthosites by picritic/troctolitic magmas or during impact melting of troctolites. The enrichment of CL objects of Dhofar 025 in incompatible trace elements suggests that the mafic component of the impact mixture may be related to the high-magnesium suite rocks, which are frequently enriched in KREEP component. The depletion of CL objects in alkalis indicates their possible relation with residual glasses formed by evaporation. However, the presence of FeO in most objects points to the insignificant extent of evaporation. Thus, evaporation cannot explain the enrichment of the CL objects in Al2O3 and other refractory components, although this process definitely took place in their formation. Their similarity to the lunar pink spinel anorthosites, whose existence was predicted from orbital data, serves as an argument in support of the possible formation of the latters by impact mixing.  相似文献   

4.
PCA (Pecora Escarpment) 02007 and Dhofar 489 are both meteorites from the feldspathic highlands of the Moon. PCA 02007 is a feldspathic breccia consisting of lithified regolith from the lunar surface. It has concentrations of both incompatible and siderophile elements that are at the high end of the ranges for feldspathic lunar meteorites. Dhofar 489 is a feldspathic breccia composed mainly of impact-melted material from an unknown depth beneath the regolith. Concentrations of incompatible and siderophile elements are the lowest among brecciated lunar meteorites. Among 19 known feldspathic lunar meteorites, all of which presumably originate from random locations in the highlands, concentrations of incompatible elements like Sm and Th tend to increase with those of siderophile elements like Ir. Feldspathic meteorites with high concentrations of both suites of elements are usually regolith breccias. Iridium derives mainly from micrometeorites that accumulate in the regolith with duration of surface exposure. Micrometeorites have low concentrations of incompatible elements, however, so the correlation must reflect a three-component system. We postulate that the correlation between Sm and Ir occurs because the surface of the Feldspathic Highlands Terrane has become increasingly contaminated with time in Sm-rich material from the Procellarum KREEP Terrane that has been redistributed across the lunar surface by impacts of moderate-sized, post-basin impacts. The most Sm-rich regolith breccias among feldspathic lunar meteorites are about 3× enriched compared to the most Sm-poor breccias, but this level of enrichment requires only a few percent Sm-rich material typical of the Procellarum KREEP Terrane. The meteorite data suggest that nowhere in the feldspathic highlands are the concentrations of K, rare earths, and Th measured by the Lunar Prospector mission at the surface representative of the underlying “bedrock;” all surfaces covered by old regolith (as opposed to fresh ejecta) are at least slightly contaminated. Dhofar 489 is one of 15 paired lunar-meteorite stones from Oman (total mass of meteorite: 1037 g). On the basis of its unusually high Mg/Fe ratio, the meteorite is likely to have originated from northern feldspathic highlands.  相似文献   

5.
An important event of K-rich granitic magmatism occurred at ca. 2.07 Ga in the Rio de la Plata Craton, Uruguay, as determined by U-Pb zircon SHRIMP geochronology of two Isla Mala Suite granitic rocks. Previous indications of Archaean ages are not confirmed. The absence of recrystallization or new zircon growth suggests that the investigated region of the craton remained unaffected by major Meso- or Neoproterozoic orogenies, although less intense deformation may have occurred and remained unregistered in the zircons.  相似文献   

6.
The plutonic rocks of the magnesian suite (Mg-suite) represent the period of lunar basaltic magmatism and crustal growth (∼4.46 to 4.1 Ga) that immediately followed the initial differentiation of the Moon by magma ocean (LMO) formation and crystallization. The volume and distribution of the Mg-suite and its petrogenetic relationship to latter stages of lunar magmatism (mare basalts) remains obscure. These plutonic rocks exhibit a range of compositions and include ultramafics, troctolites, spinel troctolites, norites, and gabbronorites. A distinguishing characteristic of this suite is that they contain some of the most magnesium-rich phases (Fo95-90) that had crystallized from lunar magmas, yet they also are significantly enriched in an incompatible element component referred to as KREEP (a late-stage product of LMO crystallization containing abundant potassium (K), rare earth elements (REE), phosphorous (P), uranium, and thorium). Ion microprobe analyses of individual mineral phases (olivine, pyroxene, and plagioclase) from the Mg-suite have shown some very unexpected characteristics that have profound implications on the origin of these basaltic magmas. Although the Mg-suite lithologies are typified by silicates with relatively high Mg′, early liquidus phases such as olivine are fairly low in Ni, Co, and Cr relative to more iron-rich olivines in the younger mare basalts. The high Y and Ti/Y in early phases such as olivine and orthopyroxene indicate that the parental basaltic melts were high in incompatible elements and contained an “ilmenite fractionation” signature. However, the Y in olivine from many of the troctolites and ultramafic lithologies are only slightly greater than that of the olivine in the mare basalts whereas olivine in the norites, gabbronorites, and Apollo 14 troctolites are exceedingly high. The KREEP component may have been added to the Mg-suite parent magmas by assimilation or mixing into the mantle source. The volume of KREEP required to be added to the parental magmas of the Mg-suite tends to favor the latter mechanism for KREEP incorporation. The extremely high abundances of KREEP in the norites and gabbronorites are a product of substantial crystallization (40% to 70%) of KREEP-enriched Mg-suite parental magmas. Basaltic magmatism associated with KREEP extended for over 1.5 billion years and appears to have changed over time. The early stages of this style of lunar magmatism (Mg-suite) appear to represent melting of early LMO cumulates with low abundances of Ni, Co, Cr, and V. Later stages of KREEP-rich basaltic magmatism seemed to clearly involve melting of a variety of LMO cumulate assemblages with higher incompatible element enrichment. It appears that the heat derived from the KREEP component was instrumental in at least initiating melting of the lunar mantle over this period of time.  相似文献   

7.
月球花岗岩--比较行星学意义   总被引：1，自引：1，他引：0  
与大陆地壳广泛出露的花岗岩不同,在月球表面仅发现了少量细小的花岗岩碎屑,此外有长英质组分以熔体包裹体形式出现于月球玄武岩的矿物中。月球花岗岩碎屑的主要矿物为石英、钾长石和钙质斜长石,具花斑状结构;含少量铁橄榄石、单斜辉石、钛铁矿、锆石、磷灰石、白磷钙矿等矿物,缺少含水矿物。月球花岗岩富K2O,富Ba,相容元素(Cr、Sc、Co、V)含量比其它月岩低,具有平坦或Ⅴ型的REE型式,负Eu异常明显。它们的化学特征可以用硅酸盐液态不混熔来解释。月球花岗岩的结晶年龄在4．4～3．9Ga间,具有至少8个年龄峰,可能代表了与花岗岩形成相关的8次独立的岩浆事件。由于月球花岗岩成因和分布对于认识月球演化和岩浆作用历史至关重要,在新一轮的深空探测中,应更加重视对月球花岗岩的研究。  相似文献   

8.
Baddeleyite: A promising geochronometer for alkaline and basic magmatism   总被引：2，自引：0，他引：2  
T. B. Bayanova 《Petrology》2006,14(2):187-200
The paper is devoted to the history of the discovery, petrological and mineralogical characterization, and U-Pb dating of baddeleyite ZrO2, which was separated from various Archean, Proterozoic, and Paleozoic rocks from Fennoscandia. The morphology of this mineral and its U-Pb age values were examined in the Archean carbonatites (2613 ± 18 Ma) of Siilinjarvi, Finland, and gabbronorite dikes (2738 ± 6 Ma) at the Kirovogorskoe deposit. U-Pb isochrons are reported for the baddeleyite-zircon pair obtained from the gabbronorites and anorthosites of the Proterozoic pyroxenite-gabbronorite-anorthosite association. The U-Pb baddeleyite dates for the early gabbronorite phase (2.5 Ga) and for a gabbronorite dike (late phase, 2.4 Ga) suggest that the basic magmatism evolved over a long time period (100 m.y.) in the Proterozoic. U-Pb dates are also reported for baddeleyite from the Paleozoic carbonatites of Kovdor, Sebljarv, and Vuorijarvi.  相似文献   

9.
北苏鲁超高压变质带前寒武纪基底研究新进展   总被引：1，自引：0，他引：1  
北苏鲁超高压变质带位于胶东牟平-即墨断裂以东的地区,其前寒武纪基底以出露新元古代的双峰式火成岩为主要特征,其主要岩石组合包括大量花岗片麻岩以及少量变（超）基性岩（榴辉岩）、变沉积岩。对花岗片麻岩、变质（超）基性岩的岩石组合、地球化学、锆石U-Pb和Lu-Hf同位素的研究表明,北苏鲁存在新太古代-古元古代的残留地壳,经历了1.8~2.2 Ga期间的岩浆-火山-变质事件;新元古代中期（0.72~0.80 Ga）与Rodinia超大陆的裂解相关的岩浆活动促使地壳的生长和再造,形成了北苏鲁的前寒武纪基底的主体;北苏鲁和苏鲁-大别造山带一样,其前寒武纪基底是扬子板块北缘的一部分,苏鲁造山带的西北边界是五莲-烟台断裂带。  相似文献   

10.
Zircon from lower crustal xenoliths erupted in the Navajo volcanic field was analyzed for U–Pb and Lu–Hf isotopic compositions to characterize the lower crust beneath the Colorado Plateau and to determine whether it was affected by ∼1.4 Ga granitic magmatism and metamorphism that profoundly affected the exposed middle crust of southwestern Laurentia. Igneous zircon in felsic xenoliths crystallized at 1.73 and 1.65 Ga, and igneous zircon in mafic xenoliths crystallized at 1.43 Ga. Most igneous zircon has unradiogenic initial Hf isotopic compositions (ɛHf=+4.1–+7.8) and 1.7–1.6 Ga depleted mantle model ages, consistent with 1.7–1.6 Ga felsic protoliths being derived from “juvenile” Proterozoic crust and 1.4 Ga mafic protoliths having interacted with older crust. Metamorphic zircon grew in four pulses between 1.42 and 1.36 Ga, at least one of which was at granulite facies. Significant variability within and between xenoliths in metamorphic zircon initial Hf isotopic compositions (ɛHf=−0.7 to +13.6) indicates growth from different aged sources with diverse time-integrated Lu/Hf ratios. These results show a strong link between 1.4 Ga mafic magmatism and granulite facies metamorphism in the lower crust and granitic magmatism and metamorphism in the exposed middle crust.  相似文献   

11.
138Ce/142Ce and 143Nd/144Nd isotope ratios of lunar samples are determined to constrain the petrogenetic differentiation and evolution of the moon. High-precision Ce-Nd isotope data, well-defined Rb-Sr isochrons, and rare earth elements (REE) abundances of lunar samples show that unexpectedly low La/Ce ratios of evolved lunar highland samples are preserved from at least 3.9 Ga. Precise analysis of REE abundances indicates that the low La/Ce ratio results from a depletion of La relative to other REE. This depletion can be seen in pristine KREEP basalts and Mg-suite rocks from 3.85 to 4.46 Ga. As REE abundances of all these samples are controlled by the presence of a KREEP component, the depletion was probably inherited from a late crystallization sequence of the lunar magma ocean related to the production of the original KREEP component.  相似文献   

12.
The composition and formation of the Earth’s primitive continental crust and mantle differentiation are key issues to understand and reconstruct the geodynamic terrestrial evolution, especially during the Archean. However, the scarcity of exposure to these rocks, the complexity of lithological relationships, and the high degree of superimposed deformation, especially with long-lived magmatism, make it difficult to study ancient rocks. Despite this complexity, exposures of the Archean Mairi Gneiss Complex basement unit in the São Francisco Craton offer important information about the evolution of South America’s primitive crust. Therefore, here we present field relationships, LA-ICP-SFMS zircon U-Pb ages, and LA-ICP-MCMS Lu-Hf isotope data for the recently identified Eoarchean to Neoarchean gneisses of the Mairi Complex. The Complex is composed of massive and banded gneisses with mafic members ranging from dioritic to tonalitic, and felsic members ranging from TTG (Tonalite-Trondhjemite-Granodiorite) to granitic composition. Our new data point to several magmatic episodes in the formation of the Mairi Gneiss Complex: Eoarchean (ca. 3.65–3.60 Ga), early Paleoarchean (ca. 3.55–3.52 Ga), middle-late Paleoarchean (ca. 3.49–3.33 Ga) and Neoarchean (ca. 2.74–2.58 Ga), with no records of Mesoarchean rocks. Lu-Hf data unveiled a progressive evolution of mantle differentiation and crustal recycling over time. In the Eoarchean, rocks are probably formed by the interaction between the pre-existing crust and juvenile contribution from chondritic to weakly depleted mantle sources, whereas mantle depletion played a role in the Paleoarchean, followed by greater differentiation of the crust with thickening and recycling in the middle–late Paleoarchean. A different stage of crustal growth and recycling dominated the Neoarchean, probably owing to the thickening of the continental crust by collision, continental arc growth, and mantle differentiation.  相似文献   

13.
报道了五台地区一个晚太古代片麻状富钾花岗岩的锆石U-Pb年龄为2509±7.4Ma.该年龄与前人获得的年代学数据一起,进一步限定了晚太古期间五台-吕梁地区与俯冲有关的弧岩浆活动的年代框架.为了探讨晚太古宙-古元古代不同类型的花岗岩的源区性质和演化,特别是当时的陆壳增生机制和古老陆壳性质,本文在锆石U-Pb年代学的基础上对相应的锆石进行了细致的Hf同位素原位测量.结果表明,晚太古代片麻状花岗岩可能代表发育在古老克拉通(东部陆块?)上的弧岩浆作用产物.碰撞后(1.8 Ga)花岗岩的形成与加厚的造山带发生垮塌性伸展有关,后者导致幔源物质底侵,并引起下地壳物质的部分熔融而形成花岗质岩石.这些壳源花岗质岩浆再以不同比例与幔源岩浆混合而形成吕梁-五台地区成分复杂的碰撞后岩浆岩(花岗质为主).锆石Hf同位素还表明,吕梁-五台地区可能存在老于2.7Ga的古老克拉通物质(2.9Ga?),虽然目前还没有发现这么老的碎屑锆石.  相似文献   

14.
KREEP Rocks   总被引：2，自引：1，他引：2  
KREEP rocks with high contents of K, REE and P were first recognized in Apollo-12 samples, and it was confirmed later that there were KREEP rock fragments in all of the Apollo samples, particularly in Apollo-12 and -14 samples. The KREEP rocks distributed on the lunar surface are the very important objects of study on the evolution of the moon, as well as to evaluate the utilization prospect of REE in KREEP rocks. Based on previous studies and lunar exploration data, the authors analyzed the chemical and mineral characteristics of KREEP rocks, the abundance of Th on the lunar surface materials, the correlation between Th and REE of KREEP rocks in abundance, studied the distribution regions of KREEP rocks on the lunar surface, and further evaluated the utilization prospect of REE in KREEP rocks.  相似文献   

15.
We provide new estimates for the abundance of heat-producing elements in the lunar mantle by using SIMS techniques to measure the concentrations of thorium and samarium in lunar pyroclastic glasses. Lunar pyroclastic glasses are utilized in this study because they represent quenched products of near-primary melts from the lunar mantle and as such, they provide compositional information about the mantle itself. Thorium and samarium were measured because: (1) Th is not significantly fractionated from Sm during partial melting of the pyroclastic glass source regions, which are dominated by olivine and pyroxene. Therefore, the Th/Sm ratios that we measure in the pyroclastic glasses reflect the Th/Sm ratio of the pyroclastic glass source regions. (2) Strong correlations between Th, U, and K on the Moon allow us to use measured Th concentrations to estimate the concentrations of U and K in the pyroclastic glasses. (3) Th, Sm, U, and K are radioactive elements and as such, their concentrations can be used to investigate heat production in the lunar mantle.The results from this study show that the lunar mantle is heterogeneous with respect to heat-producing elements and that there is evidence for mixing of a KREEP component into the source regions of some of the pyroclastic glasses. Because the source regions for many of the glasses are deep (?400 km), we propose that a KREEP component was transported to the deep lunar mantle. KREEP enriched sources produce 138% more heat than sources that do not contain KREEP and therefore, could have provided a source of heat for extended periods of nearside basaltic magmatism. Data from this study, in conjunction with models for the fractional crystallization of a lunar magma ocean, are used to show that the average lunar mantle contains 0.15 ppm Th, 0.54 ppm Sm, 0.039 ppm U, and 212 ppm K. This is a greater enrichment in radiogenic elements than some earlier estimates, suggesting a more prolonged impact of radiogenic heat on nearside basaltic volcanism.  相似文献   

16.
通过对安徽省东至县兆吉口铅锌矿区岩浆岩的LA-ICP MS锆石U-Pb年龄测定,获得矿区北部戴村花岗闪长岩体的年龄为145.5±1.3 Ma,花岗斑岩脉的年龄为143.5±4.3 Ma,细晶闪长岩脉的年龄为129.0±2.3 Ma和128.4±2.7 Ma,前两者为同一期岩浆作用的产物,后者为赋矿岩石之一,铅锌矿化与该期岩浆作用关系密切。矿区所有中酸性侵入岩体和岩脉中均发育大量继承锆石,反映有大量成岩物质来源于古老地壳基底。继承锆石核的同位素年龄集中于890～740 Ma,揭示出该区晋宁期华夏板块与扬子板块之间的构造-岩浆事件,少量大于1 000 Ma甚至2 500 Ma的锆石年龄数据反映该区可能存在早元古代甚至太古代古老陆壳基底。  相似文献   

17.
冀东地区始太古代早期—冥古宙锆石发现   总被引：1，自引：0，他引：1  
最古老大陆地壳的形成、组成和演化是阐释地球壳幔物质分异、构造演化的逻辑起点。由于长期地质作用改造,地球上保存的古老陆壳物质十分稀少,寻找和研究都极具挑战性。在冀东卢龙喇叭山地区,早期研究发现含有大量3.4~3.8 Ga碎屑锆石的铬云母石英岩,岩石特征和碎屑锆石年龄分布与迁安黄柏峪地区的铬云母石英岩十分类似。本文对喇叭山地区与铬云母石英岩共生的变质碎屑沉积岩进行了锆石定年。黑云斜长片麻岩(J2006)94个数据点分析,碎屑锆石普遍显示强烈铅丢失,数据点主要沿上交点年龄为~3.8 Ga和~3.45 Ga的两条不一致线分布。获得3084±6 Ma的变质锆石年龄。长石石英岩(J2009)68个数据点分析,除大量3.4~3.8 Ga碎屑锆石外,还发现3.92~4.0 Ga碎屑锆石(6颗)。始太古代早期—冥古宙碎屑锆石的发现表明华北克拉通无疑存在4.0 Ga以上的形成演化历史。本文新定义的曹庄-喇叭山岩系形成于3.1~3.4 Ga之间,主要由变质碎屑沉积岩和变质基性-超基性岩组成,可能代表了地幔柱与大陆壳相互作用的构造环境。根据碎屑锆石形态特征和不同样品碎屑锆石年龄分布存在差异,物源区具有近源和规模大的特征,存在3.92~4.0 Ga、3.8 Ga、3.6~3.7 Ga和3.4~3.45 Ga等不同期次花岗质岩浆作用。  相似文献   

18.
王芳  陈福坤  侯振辉  彭澎  翟明国 《岩石学报》2009,25(11):3057-3074
本文报道冀北崇礼-赤城地区晚古生代花岗岩类岩石的锆石U-Pb年龄和Sr-Nd-Hf同位素组成特征.它们出露在华北陆块北缘的构造单元内,侵位于中高级变质基底岩石红旗营子群中.锆石LA-ICP-MS定年结果表明,海流图花岗岩岩体记录了两期岩浆作用,即299±3Ma和254±11Ma;镇宁堡片麻状二长花岗岩和白花沟片麻状黑云母石英二长闪长岩分别形成于287±1Ma和252±3Ma.这些晚古生代花岗岩类岩石具有较低的初始~(87)Sr/~(86)Sr值(0.7062～0.7076)、低的ε_(Nd)(t)值(-18.1至-9.6)和古老的Nd亏损地幔模式年龄(2.49～1.87Ga).其锆石的ε~(Hf)(t)值变化在-13.2至-7.4之间,Hf平均地壳模式年龄值(T_(DM)~C)在2.15Ga至1.79Ga之间.锆石Hf同位素特征与全岩Nd同位素特征指示古老的华北陆块地壳物质是花岗岩浆的主要物源.在形成时代和地球化学特征上,崇礼-赤城地区晚古生代花岗岩与出露在东部丰宁-承德地区的花岗岩类岩石既有相似性,又有不同之处,可能代表华北陆块北缘不同构造背景下岩浆作用的产物.  相似文献   

19.
In the Kaapvaal craton of southern Africa, as well as other Archaean cratons worldwide, the progression from dominant tonalite-trondhjemite-granodiorite(TTG) to granite-monzogranite-syenogranite(GMS)rock types is interpreted to reflect progressive reworking and differentiation of the continental crust.Here we re-evaluate the early Archaean evolution of the Kaapvaal craton and propose a unified view of the plutonic and volcanic records based on elemental and isotopic(Nd, Hf) data and zircon U-Pb ages.We also report new whole-rock major and trace element analyses, zircon U-Pb ages and Hf-in-zircon analyses of igneous clasts from a conglomerate of the 3.2 Ga Moodies Group of the Barberton Greenstone Belt. Many of these clasts are derived from shallow intrusive rocks of granitic composition, which are scarcely represented in outcrop. Despite alteration, the volcanic rocks can be classified based on their trace element contents into two main groups by comparison with plutonic rocks. One group has characteristics resembling TTGs: relatively low and fractionated rare earth element concentrations with no Eu anomaly and relatively low concentrations of high field strength elements(Nb mostly ≤12 ppm). The second group has GMS-like characteristics: less fractionated REE, marked negative Eu anomalies and HFSE-increasing trends with progressing fractionation(Nb ≤ 50 ppm or more, Th up to 30-40 ppm). In addition, igneous clasts of Moodies Group conglomerate have chemical, mineralogical and isotopic characteristics that link them to GMS. New analyses of some of these clasts indicate elevated high field strength elements(Nb up to 20 ppm) and_(εHf)(t)of zircon down to -3.5. These rocks imply the presence of an already differentiated felsic crust at 3.5 Ga, which has Nd and Hf model ages indicating mantle extraction ages extending back to the Eoarchaean. The combined record of plutonic and volcanic rocks of the Kaapvaal craton provides a more complex scenario than previously suggested and indicates that TTG and GMS-like felsic magmas were emplaced broadly coevally in multiple pulses between ~3.5 Ga and 3.2 Ga.  相似文献   

20.
Zircons from 14 compositionally variable granitic rocks were examined in detail using CL image-guided micro-analysis to unravel the complex magmatic history above the southward retreating Hellenic subduction zone system in the Aegean Sea. Previously published U–Pb ages document an episodic crystallisation history from 17 to 11?Ma, with peraluminous (S-type) granitic rocks systematically older than closely associated metaluminous (I-type) granitic rocks. Zircon O- and Hf isotopic data, combined with trace element compositions, are highly variable within and between individual samples, indicative of open-system behaviour involving mantle-derived melts and evolved supracrustal sources. Pronounced compositional and thermal fluctuations highlight the role of magma mixing and mingling, in accord with field observations, and incremental emplacement of distinct melt batches coupled with variable degrees of crustal assimilation. In the course of partial fusion, more fertile supracrustal sources dominated in the earlier stages of Aegean Miocene magmatism, consistent with systematically older crystallisation ages of peraluminous granitic rocks. Differences between zircon saturation and crystallisation temperatures (deduced from zircon Ti concentrations), along with multimodal crystallisation age spectra for individual plutons, highlight the complex and highly variable physico-compositional and thermal evolution of silicic magma systems. The transfer of heat and juvenile melts from the mantle varied probably in response to episodic rollback of the subducting lithospheric slab, as suggested by punctuated crystallisation age spectra within and among individual granitic plutons.  相似文献   

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