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1.
新疆北部广泛分布晚古生代橄榄玄粗岩系火山岩和埃达克岩,它们均明显富碱,与区内同时代碱性花岗岩共同构成新疆北部富碱火成岩省。橄榄玄粗岩系火山岩和埃达克岩同位素年龄为250-280Ma,143Nd/144Nd比值高,εNd(t)值为正值,Nd模式年龄低(tDM<1.0Ga),较低而变化范围较大的(87Sr/86Sr)i(0.7040-0.710),这些特点均表明它们的源区物质为底侵幔源玄武质岩浆,并可能受到下地壳混染。结合区内地球物理资料,如加厚的地壳、复杂的莫霍面结构、高热流值及广泛分布的基性岩墙群和碱性花岗岩,均一致表明在晚古生代(晚石炭-二叠纪)新疆北部发生过较广泛的幔源玄武质岩浆底侵作用,陆壳发生了明显垂向增生。  相似文献   

2.
贺元凯  吴泰然  荆旭  罗红玲  赵磊 《岩石学报》2009,25(5):1201-1205
内蒙古四子王旗地处华北板块北缘中段,近年来在该区的早白垩世碱性玄武岩中发现了下地壳麻粒岩包体。本文研究的麻粒岩包体多数为中酸性的苏长麻粒岩,只有个别属于基性的二辉麻粒岩。麻粒岩包体的主量元素特征为SiO2含量在49.9%到69.24%之间,MgO含量在0.92%~3.56%之间,Mg#为43.0~62.9,Al2O3在13.91%~22.79%之间;稀土元素总量变化较大(∑REE在33.76×10-6~184.13×10-6之间),分配模式也不尽相同,反映这些包体来自于不同源区或经历了不同的演化过程,但总体特征是LREE相对HREE富集,大多数样品的Eu呈现明显的正异常;由于包体的来源不同或经历了不同的演化阶段,它们的微量元素特征也有很大的差异。本区研究的麻粒岩包体的地球化学特征与华北地台的麻粒岩包体和麻粒岩地体都有很大的不同,反映了地壳组成横向上的不均一性。计算得出二辉麻粒岩的平衡温压条件分别为940~947℃和1.2±0.1GPa;苏长麻粒岩的平衡温度范围为927~715℃。通过对麻粒岩包体的岩相学、平衡温压条件估算、锆石形态特征分析和锆石的LA-ICP-MS年代学研究,得到本区早白垩世时的下地壳组成和之前的演化过程:本区下地壳在约40km深处可分为上下两部分,上部主要由苏长麻粒岩组成,厚度至少为9km,下部可能由二辉麻粒岩组成;下地壳上部的苏长麻粒岩是晚太古代(2509±35Ma)的岩浆岩经历了早白垩世(约122Ma)的玄武质岩浆底侵,发生了麻粒岩相变质和部分熔融形成的。推测本区岩石圈减薄事件的起始时间为122Ma。  相似文献   

3.
地球层圈热结构特征表明,大陆地壳位于力学边界层(MBL)范围内,其热传递效应受地壳岩石热传导率制约。本文根据热传导理论,采用与前人相同的热物理参数,计算得出1200℃、500m厚玄武岩浆侵位于初始温度为500℃的地壳岩石中,由底侵作用引起地壳部分熔融产生的长英质岩浆(850℃)厚度≤250m,且产生长英质岩浆的时间限制在玄武岩侵位到地壳中后短暂的2700年内。江西南部龙南县临江地区中侏罗世余田群菖蒲组双峰式火山岩地质-地球化学特征及同位素年代学研究证实,该火山岩组合中的流纹质火山岩是拉班玄武质岩浆底侵作用的产物,符合上述数量比例和时间条件。我国东南部中生代陆壳(厚度≤50km)位于热传导起主导作用的MBL层内,由玄武岩浆底侵作用产生的长英质火成岩,在形成的时间及数量上应受地壳岩石热传导机制制约。但在我国东南部大面积出露的中生代火成岩中,花岗岩、流纹质火山岩类占90%以上,而玄武岩仅有局部零星分布,两者在数量上不匹配,在形成时间上也不一致,因而,我国东南部大面积出露的长英质火成岩可能并不是玄武岩浆底侵作用产物,其形成的热动力学背景和机制有待进一步研究。  相似文献   

4.
Lithological observations and mineralogical analyses on pyroxene and hornblende megacrysts and pyroxene and hornblende cumulates in xenoliths in the Mesozoic plutons of the Tongling region, Anhui Province, provide evidence for the magmatic underplating of mantle-derived alkali-olivine basalt at circa 140 Ma. The pyroxene and hornblende megacrysts and cumulates were formed through the AFC process at depths ranging from 27 to 35 km.  相似文献   

5.
安徽铜陵地区中生代的岩浆岩岩体中产有多种矿物巨晶和岩石包体。笔者对产于曹山辉石冈长玢岩和鸡冠石花岗闪长岩中的辉石和角闪石巨晶及其堆积岩或堆积晶进行了详细的岩相学和矿物化学研究,并在此基础上估算了矿物平衡结晶的温压条件,讨论了安徽铜陵地区中生代幔源岩浆底侵作用问题。研究表明,铜陵地区大约在140Ma以前发生了碱性橄榄玄武质岩浆的底侵作用,辉石巨晶和角闪石巨晶及其堆积晶是由底侵的碱性橄榄玄武质岩浆在28~33km深处的深位岩浆房中与下地壳发生同化混染作用形成的中基性岩浆经结晶分异作用形成的,而辉石堆积岩是由中基性岩浆在19~21km深处的浅位岩浆房中与中地壳的浅变质岩系发生同化混染作用形成的中性岩浆经结晶分异作用形成的。  相似文献   

6.
黄小龙  徐义刚 《地球化学》2002,31(5):443-454
女山麻粒岩包体的Mg#值总体较低(28-51),在化学成分上主要表现为偏中性,没有明显的辉石、石榴子石或Fe-Ti氧化物的堆晶作用,但部分有斜长石的堆晶作用,略具Eu,Sr正异常,部分则有斜长石的分离结晶,略具Eu,Sr负异常。另外,女山麻粒岩包体亏损Rb,Th,Nb,Ta,富集Ba,LREE,Zr,Hf等,具较高的Nb/Ta,Zr/Hf比值,麻粒岩相变质作用过程造成了Rb的强烈亏损,但对Nb,Ta,U,Th的影响较弱。女山麻粒岩包体的形成过程中存在岩浆的结晶分异作用,通过分离结晶混染作用模型(AFC)模拟,表明华北克拉通太古宙基性火山岩和中必一麻粒岩分别为较可能的初始岩浆和混合源,初始岩浆经过10%-40%的AFC可以形成女山麻粒岩包体的微量元素组成。总体上,女山麻粒岩包体的主元素、微量元素特征均相似于华北克拉通太古宙中性麻粒岩、华北克拉通下地壳平均值,而与扬子克拉通下地壳平均值得明显差别。因此女山地区下地壳可能仍属华北克拉通的一部分,因而支持Li(1994)的碰撞拆离模式。  相似文献   

7.
安徽沙溪斑岩铜(金)矿床成岩成矿热历史探讨   总被引:11,自引:1,他引:11  
徐文艺  徐兆文 《地质论评》1999,45(4):361-367
采用Rb-Sr方法,测定沙溪斑岩铜(金)矿成矿岩体年龄为143.37±5.17Ma;采用^40Ar/^39Ar快中子活化定年法,测定沙溪斑岩铜(金)矿成矿年龄为123.6±0.7Ma;成矿岩体从固结成岩到成矿经历了20Ma。根据Rb-Sr和K-Ar同位素体系封闭温度的不同,估算出沙溪岩浆热液成矿系统热衰减速率平均为20℃/Ma。单一的由成矿小岩体所提供的热能难以维持如此长时间热液成矿活动,矿区晚期  相似文献   

8.
Zircons from granulite xenoliths entrained in a Late Cretaceous mafic dike in the Jiaodong Peninsula, North China Craton (NCC), show three distinct U-Pb age populations. Part of the old zircon grains yield discordant data that project to ages of about 2.4 to 2.5 Ga, a few grains indicate growth at about 2.0 Ga and a third group yield Cretaceous ages with peaks at 120 and 90 Ma. The oldest zircons give Hf TDM model ages of 2.6-2.8 Ga. These results demonstrate the existence of original Archean lower crust in the Jiaodong region. Zircons of 2.0 Ga have similar Hf TDM model ages as the Neoarchean-Paleoproterozoic grains, suggesting that these zircons were products of metamorphic recrystallization due to thermal event without juvenile input. Early Cretaceous zircons yield εHf(t) values of − 21 to − 12 and Late Cretaceous zircons large variable εHf(t) from + 4 to − 50. These data suggest that magmatic underplating occurred in the Neoarchean to Earliest Proterozoic lower crust of the NCC, both in the Early and Late Cretaceous. It is suggested that the Mesozoic magma underplating, which also provided the heat source for the voluminous Mesozoic magmatism in the NCC, significantly modified the composition of the Archean to Paleoproterozoic lower crust of the NCC.  相似文献   

9.
Petrographical and geochemical studies were carried out of the Neoproterozoic carbonates from northern Anhui Province, China. These carbonates can be subdivided into two types, including purified limestone (PL) and mixed limestone and siliclastic rocks (MLS). PLs are low in Si, Al, Zr and total REE but high in CaO compared with MLSs. Correlations between Zr, Fe2O3, P2O5 and total REE are also less remarkable, indicating that they were not obviously contaminated by continental materials but directly precipitated from seawater. So they can be used for the inversion of paleo-seawater REE patterns. The inversed results indicated that the REE pattern of Neoproterozoic seawater was similar to that of the modern seawater except for the lack of Ce depletion, which was probably affected by the reduced hydrothermal solution. In combination with previous studies, the geochemical trending of these carbonatic samples towards the continental arc in the La- Th-Sc and Th-Sc-Zr/10 diagrams is interpreted as the contamination of arc volcanism. The tectonic background of the southern margin of the North China Craton in the Neoproterozoic was probably related to the Grenville orogeny during the convergence of the Rodinia supercontinent, rather than an intra-plate environment.  相似文献   

10.
广东英德白沙佛冈花岗岩风化壳地下水中的稀土元素   总被引:3,自引:3,他引:3  
广东省英德市白沙镇花岗岩风化壳地下水主要属于HCO3-Ca·Na型。水样的pH值在4.3~6.6之间。与世界其他花岗岩地区地下水相比,白沙地区花岗岩风化壳地下水的稀土元素(REE)总浓度偏高,在1.1~114μg/L之间,变化范围较大。地下水中的REE浓度与水的pH值呈弱的负相关关系,并且受母岩风化程度的影响。由于pH值、氧化还原状态及水中主要离子组成的差异,使得可溶态REE在水中的赋存状态存在差异。地下水与母岩、风化壳REE的PAAS标准化分布模式基本相似,呈LREE富集型,母岩中无Ce异常,而水样中有Ce负异常,表明Ce在水岩作用过程中发生了明显的分异。  相似文献   

11.
Ten representative limestone samples [six from Wangshan Formation (WL) and the other four from Gou- hou Formation (GL)] were collected from outcrops of the northern Anhui Province, and REE concentrations in lime- stone were measured by ICP-MS. The depositional environment and source of the REE were analyzed, and the re- sults implys that the total REE of samples are low compared with the recent marine sediments, which range from 9.61 to 54.18 mg/kg and an average of 22.93 mg/kg. The PAAS-normalized REE+Y patterns of limestone are char- acterized by (1) light REE depletion in WL and enrichment in GL with the values of (Nd/Yb)sN ranging form 0.65 to 0.91 and 1.12 to 1.46, respectively; (2) light negative Ce anomaly (0.85-1.02) and positive La anomaly (0.92-1.27); and (3) both Eu (0.91-1.23) and Y (1.42-2.38) expressing light positive anomaly. The character indicated that the depositional environment was oxygenated, with infection by the hydrothermal activity and contamination of detritus.  相似文献   

12.
Both adakitic and shoshonitic igneous rocks in the Luzong area, Anhui Province, eastern China are associated with Cretaceous Cu–Au mineralization. The Shaxi quartz diorite porphyrites exhibit adakite-like geochemical features, such as light rare earth element (LREE) enrichment, heavy REE (HREE) depletion, high Al2O3, MgO, Sr, Sr / Y and La / Yb values, and low Y and Yb contents. They have low εNd(t) values (− 3.46 to − 6.28) and high (87Sr / 86Sr)i ratios (0.7051–0.7057). Sensitive High-Resolution Ion Microprobe (SHRIMP) zircon analyses indicate a crystallization age of 136 ± 3 Ma for the adakitic rocks. Most volcanic rocks and the majority of monzonites and syenites in the Luzong area are K-rich (or shoshonitic) and were also produced during the Cretaceous (140–125 Ma). They are enriched in LREE and large-ion lithophile elements, and depleted in Ti, and Nb and Ba and exhibit relatively lower εNd(t) values ranging from − 4.65 to − 7.03 and relatively higher (87Sr / 86Sr)i ratios varying between 0.7057 and 0.7062. The shoshonitic and adakitic rocks in the Luzong area have similar Pb isotopic compositions (206Pb / 204Pb = 17.90–18.83, 207Pb / 204Pb = 15.45–15.62 and 208Pb / 204Pb = 38.07–38.80). Geological data from the Luzong area suggest that the Cretaceous igneous rocks are distributed along NE fault zones (e.g., Tanlu and Yangtze River fault zones) in eastern China and were likely formed in an extensional setting within the Yangtze Block. The Shaxi adakitic rocks were probably derived by the partial melting of delaminated lower crust at pressures equivalent to crustal thickness of > 50 km (i.e., 1.5 GPa), possibly leaving rutile-bearing eclogitic residue. The shoshonitic magmas, in contrast, originated mainly from an enriched mantle metasomatized by subducted oceanic sediments. They underwent early high-pressure (> 1.5 GPa) fractional crystallization at the boundary between thickened (> 50 km) lower crust and lithospheric mantle and late low-pressure (< 1.5 GPa) fractional crystallization in the shallow (< 50 km) crust. The adakitic and shoshonitic rocks appear to be linked to an intra-continental extensional setting where partial melting of enriched mantle and delaminated lower crust was probably controlled by lithospheric thinning and upwelling of hot asthenosphere along NE fault zones (e.g., Tanlu and Yangtze River fault zones) in eastern China. Both the shoshonitic and adakitic magmas were fertile with respect to Cu–Au mineralization.  相似文献   

13.
孙林华  桂和荣 《地球学报》2011,32(5):523-532
在对安徽省北部宿州新元古代史家组砂岩地球化学特征分析的基础上,结合前人发表的淮南和凤阳新元古代刘老碑组页岩的地球化学数据,对它们的物源和构造背景进行了对比讨论.结果表明:刘老碑组页岩的地球化学特征表明其来自于中-酸性火成岩高程度的首次风化,而史家组砂岩则来自于经历了再循环的古老地壳物质较低程度的风化.刘老碑组页岩相对史...  相似文献   

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