全球苦橄岩与太古宙科马提岩对比：全数据模式的启示

苦橄岩和科马提岩都是富镁的超镁铁质火山岩,早先,学术界大多关注它们之间的相似性,而对于它们之间的差异性很少强调。于是认为二者的地球化学性质近似,成因类似,形成条件类似。本文采用全数据模式的研究方法,从数据库收集了全球太古宙全部科马提岩和后太古宙全部苦橄岩数据,对比的结果表明,太古宙科马提岩与后太古宙苦橄岩完全不同,它们之间几乎没有可比性。科马提岩与苦橄岩,不仅地球化学特征不同,而且成因不同,形成条件不同,产出时代不同,源区组成也不同。这种不同,反映了太古宙和后太古宙不可能属于同样的构造体制。太古宙是火球时代, 地球异常的热, 主导的可能是静止盖幔构造（stagnant lid tectonics）;后太古宙是热球时代,地球相对冷了许多,主导的是板块构造（plate tectonics）。科马提岩在太古宙广泛出露,无需地幔柱模式;而苦橄岩在后太古宙很少出露,才真正需要地幔柱模式。     

科马提岩与苦橄岩的区别及对若干晚古生代“科马提岩”的质疑

以往学术界更多的关注科马提岩和苦橄岩的相似性,忽略其差异。通过全数据模式,采集数据库内全球的太古宙科马提岩、后太古宙低/高钛苦橄岩数据,对比三者之间的差异发现,科马提岩更富MgO、Cr、Ni、Cs、Pb、Co和Zn,其次为低钛苦橄岩(除Co和Zn),其余主量、微量元素的含量由高至低依次为高钛苦橄岩、低钛苦橄岩、科马提岩。依据元素间的差异(如Cr/Ga、MgO/Ga、MnO/Zr、Cr/Zr等),采用密度分布函数(Density Distribution)在Matlab软件中绘制出可有效区分3类岩石的等密度判别图,并用该图对若干晚古生代"科马提岩"的岩性重新厘定。结合岩相学和地球化学特征研究表明,晚古生代"科马提岩"中,印度东部为高钛苦橄岩,越南为化学成分与科马提岩类似的低钛苦橄岩,印度拉达克地区为低钛苦橄岩。     

Re-Os同位素对峨眉山大火成岩省成因制约的探讨

峨眉山大火成岩省（ELIP）主要由玄武岩、玄武质火山碎屑岩及少量的苦橄岩（包括越南的科马提岩）、长英质岩石以及层状岩体和岩墙组成,其物质来源直接关系到其成因是否与地幔柱活动有关。Re-Os同位素体系是地核、地幔和地壳物质的最佳示踪剂。前人对ELIP内的Re-Os同位素研究表明,低Ti玄武岩的Os含量为0．006×10^-9-0．40010^-9,^187Os／^188Os初始值为0．1371～1．403,并提出其与地幔柱活动有关;而高Ti玄武岩的Os含量为0．00410^-9～0．56010^-9,^187Os／^188Os初始值为0．1271～5．19,认为起源于大陆岩石圈地幔或地幔柱上升过程中受到大量岩石圈地幔“混染”（xu JF et al．,2007）;科马提岩的0s含量为1．2410^-9～7．0010^-9,^187Os／^188Os初始值为0．1251～0．1261,苦橄岩的Os含量为0．3210^-9～2．32910^-9,^187Os／^188Os初始值为0．1233～0．1266,指示苦橄岩和科马提岩均来自亏损地幔源区（Hanski et al．,2004;陈雷等,2007）。本文利用Os含量最低、^187Os／^188Os最高的高Ti玄武岩作为地壳端员,用铁质陨石、原始上地幔（PUM）和亏损地幔（DMM）作为地核和各种地幔端员,分别做二元混合计算,结果显示绝大多数玄武岩和所有苦橄岩及科马提岩均落在地壳和DMM混合曲线附近,并且邻区特提斯洋地幔岩与DMM具有相近的Os含量和^187Os／^188Os组成,据此推测峨眉山火成岩的形成与特提斯洋的活动有关,主要受控于地壳和亏损地幔的相互作用。     

峨眉山大火成岩省中高Os苦橄岩的发现及地质意义

本文对峨眉山大火成岩省中苦橄岩及其共生的玄武岩进行了铂族元素(PGE)分析,结果表明苦橄岩比玄武岩的PGE含量要高至少一个数量级,并且具有明显高的Os含量,不仅比熔融程度最高的科马提岩要高,而且比原始地幔还要高,另外,还显示出超球粒陨石的Os/Ir比值(2.84～3.88)。其高的Os/Ir比值可能与岩浆上升过程中混入黑色页岩有关。部分熔融计算表明,含有0.01%硫化物的原始地幔 0.5%的外核在7%的熔融程度下,然后又被约10%的黑色页岩混染可以模拟原始岩浆的PGE含量。其Os含量及其他地球化学特征与其同时代的西伯利亚暗色岩系的相似性可能暗示了这两个大火成岩省来自于同一个起源于核-幔边界的超级地幔柱。另外,还根据苦橄岩和玄武岩PGE的含量估算了该地区PGE的成矿潜力。     

云南丽江苦橄岩Re-Os同位素地球化学初步研究

报导了云南丽江地区大具和仕满剖面12个苦橄岩和6个玄武岩的Re，Os含量和Os同位素组成。苦橄岩和玄武岩具有明显不同的Re-Os体系的特征。苦橄岩具有高的Os元素丰度［(1.5~3)×10-9］和低的Re元素丰度（<0.05×10-9）；共生的玄武岩具有低的Os元素丰度（<0.5×10-9）和相对高的Re元素丰度（<0.8×10-9）；苦橄岩具有低放射成因的 187Os/188Os 比值（0.123 3~0.126 6），而玄武岩具有高放射成因的187Os/188Os比值（0.133 8~0.157 7）。苦橄岩的Re-Os同位素特征与越南西北部二叠—三叠纪科马提岩具有低放射成因Os同位素特征相似，而玄武岩的Re-Os同位素特征与峨眉山大火成岩省（LIP）其他地区玄武岩的高放射成因的Os同位素特征相似。苦橄岩的Re-Os同位素特征表明，形成峨眉山LIP的地幔柱可能来自对流上地幔而不是深部的核-幔界面。换言之，峨眉山LIP的形成受控于岩石圈地幔过程而不是地幔柱过程。     

青藏高原东南木里地区二叠纪苦橄岩的Os-Sr-Nd同位素地球化学研究

本文报道了在青藏高原东南木里地区发现的二叠纪苦橄岩和与其共生玄武岩的主微量元素地球化学特征以及Os-Sr-Nd同位素组成。苦橄岩和与其共生玄武岩受地壳混染作用影响较小。根据苦橄岩的Ti/Y比值和初始的Os同位素组成,将木里苦橄岩分为两类:高Ti/Y型苦橄岩和低Ti/Y型苦橄岩,其中高Ti/Y型苦橄岩具有高的γ_(Os)= 5.3～ 10.7和ε_(Nd)= 5.9～ 6.4,与全球典型洋岛玄武岩的Os和Nd同位素组成接近,代表了地幔柱源区的同位素特征;而低Ti/Y型苦橄岩具有低的γ_(Os)=-4.1～ 1.2和ε_(Nd)= 3.2～ 5.0,可能表明受到了SCLM(大陆岩石圈地幔)源区物质的混染。与其共生的玄武岩具有低的γ_(Os)=-3.5～-1.6和ε_(Nd)=-0.6～ 0.7,表明其来自于不同于低Ti/Y型苦橄岩也有异于高Ti/Y型苦橄岩的地幔源区,但是也可能受到了SCLM物质的混染。基于Nd-Os同位素的地幔柱与SCLM的二端元混合模型显示:低Ti/Y型苦橄岩可能是SCLM物质组分与地幔柱起源的苦橄质原始岩浆混合形成的;与苦橄岩共生的玄武岩可能是由地幔柱来源的玄武质岩浆与SCLM小比例熔融的熔体混合形成的。     

四川攀枝花新元古代苦橄质岩脉的铂族元素地球化学特征

四川攀枝花地区出露有新元古代苦橄质岩脉。本文研究表明这些苦橄质岩脉的铂族元素(PGE)含量较高(19.7~29.0 ng/g),原始地幔标准化后的PGE分布模式呈Pt-Pd富集型,Pd/Ir值(5.64~11.33)与高镁玄武岩和科马提岩相似。同时,这些岩石显示在形成过程中没有经历硫化物和PGE合金矿物的熔离,其原始岩浆起源于地幔较高程度的部分熔融,可能与地幔柱的影响有关。通过扣除铬尖晶石和橄榄石结晶分异对PGE造成的影响,得到原始岩浆的PGE组成特征为Ir、Ru、Rh相对于Pt、Pd明显亏损,在源区已无硫化物存在的条件下,这很可能是由于地幔部分熔融过程中有IPGE合金矿物残留在地幔源区。攀枝花地区苦橄质岩脉可能与该地区冷水箐Cu-Ni硫化物矿床具有相似的原始岩浆组成。     

四川杨柳坪Cu—Ni—PGE矿区基性—超基性岩的地球化学特征及其含矿性

杨柳坪矿区的基性-超基性岩主要呈层状产出，可以分为2类，即含矿的强蚀变超基性岩和不含矿的弱蚀变或未明显蚀变的基性岩，前者属于苦橄岩并具有科马提岩的地球化学特征（不具鬣刺结构，但可称为科马提质的苦橄岩），同时还具有高H2O^ 、高CO2的特点，并且H2O^ 、CO2越高矿化越强，表明成矿作用与热液蚀变有关，后者在地质特征和地球化学特征上与峨眉山玄武岩相似，成矿元素含量正常。     

二滩玄武岩的水含量：峨眉山大火成岩省地幔源区水含量的区域性分布特征

近期对科马提岩以及许多大火成岩省中的苦橄岩进行的水含量分析以及地幔潜能温度的研究表明,无论是太古宙还是显生宙的大火成岩省的形成都和含水的地幔柱有关。晚二叠纪的峨眉山大火成岩省（ELIP）位于扬子克拉通西缘,目前主流观点认为其是由地幔柱形成的。前人根据大量的岩石地球化学工作将ELIP分为西区、中区和东区;证明了位于西区的大理、宾川的苦橄岩和玄武岩地幔源区的水含量高于2500×10^-6。然而对于其他区域玄武岩源区的含水性还不清楚。文章以位于中区的二滩剖面底部高钛型玄武岩为研究对象,采用单斜辉石斑晶反演的方法研究恢复了其原始岩浆的水含量。结果表明,单斜辉石斑晶水含量范围为76×10^-6 ~424×10^-6, 对应的平衡熔体水含量为3.01 wt%。在考虑分离结晶影响后,恢复的原始岩浆水含量达到2.71±0.95 wt%。该水含量略低于大理苦橄岩水含量,与宾川苦橄岩相当。而计算的地幔源区水含量最低估计为1357×10^-6,该值低于大理、宾川苦橄岩的源区水含量,但仍显著高于正常洋中脊玄武岩和洋岛玄武岩源区。ELIP中不同区域的苦橄岩和玄武岩都存在高水含量,这表明在ELIP的形成和演化过程中水都扮演了很重要的角色。     

丽江玄武岩水含量：对峨眉山大火成岩省源区水含量分布特征的启示

近年来对太古宙科马提岩和显生宙大火成岩省中苦橄岩的水含量、地幔潜热、源区成分等研究表明,这些短时间内喷出巨量岩浆的地表过程都与水化的地幔柱有关。峨眉山大火成岩省位于扬子板块西部,是我国被公认的大火成岩省之一。前人从地球化学的角度将其分为西、中、东三区;并通过对西区丽江、永胜、宾川、大理苦橄岩和中区二滩玄武岩的水含量分析,发现形成峨眉山大火成岩省的地幔柱可能自喷发初期就已普遍存在强烈的水化,且该特征持续至喷发中晚期。然而前人的研究着重于苦橄岩,对作为大火成岩省主体部分的玄武岩研究甚少。本文以位于西区的仕满、大具剖面中的高Ti/Y玄武岩为研究对象,采用单斜辉石斑晶反演原始熔体水含量的方法,得到仕满、大具玄武岩原始熔体的水含量下限分别为1.15%和0.83%,该水含量略低于丽江苦橄岩水含量。而计算出的源区最低水含量分别为1380×10^-6和1245×10^-6,与二滩玄武岩相当。结合前人报道的数据,本次工作的结果证明了峨眉山大火成岩省的地幔柱水化现象普遍且长期存在,地幔柱内部的热化学组成是不均一的,且其热化学结构是随着时间而发生变化的。本次工作还暗示...     

Os isotopic constraints on Archean mantle dynamics

The Re-Os isotopic systematics of two ca. 2.7-Ga komatiite flows from Belingwe, Zimbabwe are examined. Rhenium and Os concentrations in these rocks are similar to concentrations in other Archean, Proterozoic, and Phanerozoic komatiites. Despite the excellent preservation of primary magmatic minerals, the Re-Os systematics of whole-rock samples of the komatiites show open-system behavior. Consistent model ages for several whole-rock samples suggest a disturbance to the system during the Proterozoic. Despite the open-system behavior in the whole rocks, Re-Os systematics for concentrates of primary magmatic olivine and spinel indicate generally closed-system behavior since the magmatic event that produced the rocks. Regression of the data for the mineral concentrates yields an age of 2721 ± 21 Ga, which is consistent with Pb-Pb and Sm-Nd ages that have been previously reported for the komatiites (Chauvel et al., 1993), and an initial ¹⁸⁷Os/¹⁸⁸Os ratio of 0.11140 ± 84 (γ_Os = +2.8 ± 0.8).The 2 to 3% enrichment in ¹⁸⁷Os/¹⁸⁸Os ratio of the mantle source of the komatiites, relative to the chondritic composition of the contemporaneous convecting upper mantle, most likely reflects either the incorporation of substantially older (≥ 4.2 Ga), Re-rich recycled mafic crust into the mantle source of the komatiites or the contribution of suprachondritic Os to the source from the putative ¹⁸⁷Os-enriched outer core. The former interpretation would indicate the Hadean formation and recycling of mafic crust. The latter interpretation would require early formation of a substantial inner core followed by upwelling of a mantle plume from the core-mantle boundary, at least as far back as the Late Archean. Either interpretation requires large-scale mantle convection during the first half of Earth history.     

Mafic-ultramafic magmatism of the Early Precambrian (from the Archean to Paleoproterozoic)

Compositional evolution of the Archean mafic-ultramafic volcanics is considered in comparison with evolution of the Paleoproterozoic volcanism using available data on the Baltic shield, Pilbara (Australia) and Superior (Canada) cratons, and the Isua greenstone belt (Greenland). The Archean volcanics of mantle origin are of two major types, represented (a) by komatiite-basaltic complexes (komatiites, komatiitic and tholeiitic basalts) and (b) by geochemical analogs of boninites (GAB) and siliceous high-Mg series (SHMS) of volcanic rocks. As is established, the komatiitic and GAB volcanism ceased in the terminal Archean, whereas the SHMS rocks prevailed in the Paleoproterozoic to become extinct about 2 Ga ago in connection with transition to the Phanerozoic type of tectonomagmatic activity. Geochemical trends of mafic-ultramafic associations occurring in the considered cratons are not uniform, being of particular character to certain extent. With transition from the Paleo- to Neoarchean, rock associations of both types reveal a minor increase in Ti and Fe contents. Comparatively high Fe₂O_3tot TiO₂, and P₂O₅ concentrations (maximal ones in the Archean), which are characteristic of the Neoarchean (2.75–2.70 Ga) basalts from the Superior and Pilbara cratons or the Baltic shield, represent a result of relatively high-Ti intracratonic magmatic activity that commenced in that period practically for the first time in the Earth history. This magmatic activity of the Neoarchean was not as intense as the high-Mg basaltic volcanism, and the absolute maximum in concentrations of the above components was attained only 2.2–1.9 Ga ago, at the time of appearance in abundance of Fe-Ti picrites and basalts typical of the Phanerozoic intraplate magmatism. The Archean volcanic complexes demonstrate gradual secular increase in concentrations of incompatible elements (LREE inclusive) and growth of Nb/Th ratio that apparently reflected the progressing influence of mantle plumes. In the early Paleoproterozoic (2.5–2.35 Ga), values of that ratio considerably declined in the SHMS rocks and then quickly grew in the Middle Paleoproterozoic volcanics (2.2–1.9 Ga) to attain finally the values typical of the Phanerozoic magmas associated in origin with mantle plumes. The ?_Nd(T) parameter was decreasing with time from positive values in the Paleoarchean to negative ones in the SHMS rocks of the Paleoproterozoic most likely in response to grown proportion of ancient crustal material in magmatic melts. Since the mid-Paleoproterozoic, the ?_Nd(T) values turn in general into positive again reflecting change in the character of magmatic activity: the SHMS melts gave place at that time to the Fe-Ti picrite-basaltic magmas. The primary crust of the Earth was presumably of sialic composition and originated during solidification from the bottom upward of the global magma ocean a few hundreds kilometers deep, when most fusible components migrated up to the surface to form there the granitic crust. Geological history of the Earth commenced at the appearance time of granite-greenstone terranes and granulite belts separating them, the first large tectonic structures formed under influence of raising mantle superplumes.     

Geochemical evolution of magmatism in Archean granite-greenstone terrains

Evolution of Archean magmatism is one of the key problems concerning the early formation stages of the Earth crust and biosphere, because that evolution exactly controlled variable concentrations of chemical elements in the World Ocean, which are important for metabolism. Geochemical evolution of magmatism between 3.5 and 2.7 Ga is considered based on database characterizing volcanic and intrusive rock complexes of granite-greenstone terrains (GGT) studied most comprehensively in the Karelian (2.9–2.7 Ga) and Kaapvaal (3.5–2.9 Ga) cratons and in the Pilbara block (3.5–2.9 Ga). Trends of magmatic geochemical evolution in the mentioned GGTs were similar in general. At the early stage of their development, tholeiitic magmas were considerably enriched in chalcophile and siderophile elements Fe₂O₃, MgO, Cr, Ni, Co, V, Cu, and Zn. At the next stage, calc-alkaline volcanics of greenstone belts and syntectonic TTG granitoids were enriched in lithophile elements Rb, Cs, Ba, Th, U, Pb, Nb, La, Sr, Be and others. Elevated concentrations of both the “crustal” and “mantle-derived” elements represented a distinctive feature of predominantly intrusive rocks of granitoid composition, which were characteristic of the terminal stage of continental crust formation in the GGTs, because older silicic rocks and lithospheric mantle were jointly involved into processes of magma generation. On the other hand, the GGTs different in age reveal specific trends in geochemical evolution of rock associations close in composition and geological position. First, the geochemical cycle of GGT evolution was of a longer duration in the Paleoarchean than in the Meso-and Neoarchean. Second, the Paleoarche an tholeiitic associations had higher concentrations of LREE and HFSE (Zr, Ti, Th, Nb, Ta, Hf) than their Meso-and Neoarchean counterparts. Third, the Y and Yb concentrations in Paleoarchean calc-alkaline rock associations are systematically higher than in Neoarchean rocks of the same type, while their La/Yb ratios are in contrast lower than in the latter. These distinctions are likely caused by evolution of mantle magmatic reservoirs and by changes in formation mechanisms of silicic volcanics and TTG granitoids. The first of these factors was likely responsible for appearance of sanukitoid magmatic rocks in the Late Mesoarchean. Representative database considered in the work includes ca. 500 precision analyses of Archean magmatic rocks.     

http://www.sciencedirect.com/science/article/pii/S1674987116000311

Greenstone basalts and komatiites provide a means to track both mantle composition and magma generation temperature with time.Four types of mantle are characterized from incompatible element distributions in basalts and komatiites:depleted,hydrated,enriched and mantle from which komatiites are derived.Our most important observation is the recognition for the first time of what we refer to as a Great Thermal Divergence within the mantle beginning near the end of the Archean,which we ascribe to thermal and convective evolution.Prior to 2.5 Ga,depleted and enriched mantle have indistinguishable thermal histories,whereas at 2.5-2.0 Ga a divergence in mantle magma generation temperature begins between these two types of mantle.Major and incompatible element distributions and calculated magma generation temperatures suggest that Archean enriched mantle did not come from mantle plumes,but was part of an undifferentiated or well-mixed mantle similar in composition to calculated primitive mantle.During this time,however,high-temperature mantle plumes from dominantly depleted sources gave rise to komatiites and associated basalts.Recycling of oceanic crust into the deep mantle after the Archean may have contributed to enrichment of Ti,Al,Ca and Na in basalts derived from enriched mantle sources.After 2.5 Ga,increases in Mg~# in basalts from depleted mantle and decreases in Fe and Mn reflect some combination of growing depletion and cooling of depleted mantle with time.A delay in cooling of depleted mantle until after the Archean probably reflects a combination of greater radiogenic heat sources in the Archean mantle and the propagation of plate tectonics after 3 Ga.     

Late Archaean Ti–rich,Al–depleted komatiites and komatiitic volcaniclastic rocks from the Murchison Terrane in Western Australia

Greenstone belts in the northern Murchison Terrane of the Yilgarn Craton contain an extensive suite of 2.9–3.0 Ga, porphyritic komatiites and komatiitic volcaniclastic rocks. These unusual Ti–rich Al–depleted komatiites have been sampled at Gabanintha and are characterised by higher incompatible‐element abundances than most suites of Barberton‐type Al–depleted komatiites. They form a petrogenetically related group with similar Ti– and incompatible‐element‐rich, Al–depleted porphyritic komatiites and komatiitic volcaniclastic rocks from Karasjok in Norway, Dachine in French Guiana and Steep Rock‐Lumby Lake in Canada (here called Karasjok‐type komatiites). Their Al–depletion results from magma generation at depths of >250 km in the presence of residual majorite‐garnet. The porphyritic textures and abundance of amygdales and volcaniclastic rocks typical of this type of komatiite are features of hydrous ultramafic magmas. The incompatible‐element‐rich ultramafic rocks from Dachine contain diamonds that were most likely picked up as parent magmas interacted with mantle lithosphere that had been hydrated and chemically modified. Consequently the interaction of Karasjok‐type komatiite magmas with thick, island arc or continental mantle lithosphere may have resulted in their elevated water and incompatible‐element contents. The occurrence of Karasjok‐type komatiite lavas and volcaniclastic rocks in the northern Murchison Terrane suggests that during the Late Archaean that terrane had a hydrated, metasomatised or subduction‐modified mantle lithosphere.     

扬子岩浆岩带东段基性岩地球化学

长江中下游中生代岩浆岩带称为扬子岩浆岩带。该带岩浆岩属高钾钙碱性岩系和橄榄安粗岩系,共基性端员玄武岩和辉长岩高钾富碱,硅弱不饱和,富集Ｒｂ、Ｂａ、Ｔｈ、Ｋ、ＬＲＥＥ等强不相容元素,强烈亏损Ｃｒ、Ｎｉ等强相容元素;ＲＥＥ球粒陨石标准化曲线为右倾型,在Ｌａ／Ｓｍ－Ｌａ图上排列成一斜线,是地幔不同程度部分熔融为主的产物。基性岩εＮｄ较高,Ｉｓｒ较低,在εＮｄ－ＩＳｒ图上沿地幔排列及其延长线分布,略向右漂     

Geochemistry of mafic–ultramafic magmatism in the Western Ghats belt (Kudremukh greenstone belt), western Dharwar Craton,India: implications for mantle sources and geodynamic setting

Western Ghats Belt of western Dharwar Craton is dominated by metavolcanic rocks (komatiites, high-magnesium basalts (HMBs), basalts, boninites) with occasional metagabbros. This rock-suite has undergone post-magmatic alteration processes corresponding to greenschist- to lower-amphibolite facies conditions. Komatiites are Al-depleted, characterized by lower Al₂O₃/TiO₂ and high CaO/Al₂O₃. Their trace element distribution patterns suggest most of the primary geochemical compositions are preserved with minor influence of post-magmatic alteration processes and negligible crustal contamination. Chemical characteristics of Al-depleted komatiites imply their derivation from deeper upper mantle with/without garnet involvement. HMBs and basalts are differentiated based on their magnesium content. Basalts and occasionally associated gabbroic sills have similar geochemical characteristics. HMB are characterized by light rare earth element (LREE) enrichment, with significant Nb–Ta and Zr negative anomalies. Basalts and associated gabbros display tholeiitic affinity, with LREE-enriched to slightly fractionated heavy rare earth element (HREE) patterns. Boninites are distinctive in conjunction of low abundances of incompatible elements with respect to the studied komatiites. Chondrite-normalized REE patterns of boninites show relative enrichment in LREE and HREE with respect to MREE. Prominent island arc signatures are evident in HMB, basalts, boninites, and gabbros in terms of their Nb–Ta and Zr–Hf negative anomalies, LREE enrichment and HFSE depletion. It is suggested that these HMB–basalts (associated gabbros)–boninites are the products of arc magmatism. Their REE chemistry attests to a gradual transition in melting depth varying between spinel and garnet stability field in an arc regime. The close spatial association but contrasting elemental characteristics of komatiites and HMB–basalts–boninites can be explained by a plume-arc model, in which the ~3.0 Ga komatiites are considered to be the products of plume volcanism in an oceanic setting, while the HMB, basalts, boninites, and associated gabbros were emplaced in a continental margin setting around 2.8–2.7 Ga.     

Physicochemical processes involved in Cenozoic volcanism in eastern China

Eastern China is a Cenozoic composite volcanic rock province, where volcanic rocks of the tholeiite series, calc-alkali series, Hy-norm-bearing olivine basalt series, Na-alkali series and K-alkali series coexist. Eastern China is separated into the northern and southern volcanic rock regions by the Changzhou-Yueyang old deep fault. Magma generation and magmatic activities in the northern region were controlled by the mantle uplift and old deep faults. These old deep faults were revived and some of them were changed into a multiple rift system due to back-arc expansion. The Bohai Sea depression is situated at the intersection of the Lujiang-Tancheng-Shenyang-Mishan and Zhangjiakou-Tianjin uplift belts of the upper mantle. Eogene (71.5-28.5 Ma) tholeiites largely occur in the central part of the mantle uplift; the well developed Neogene (23.8-2.6 Ma) alkali olivine basalts are distributed in the outer lane of the former and the Quaternary (1.48 Ma-recent) peralkali volcanic rocks are far away from them. In the southern region magma generation and magmatic activities were controlled mainly by plate subduction and three sets of old deep faults. Studies of incompatible elements and REE show that the degree of enrichment of incompatible elements and LREE increases with decreasing age, increasing source depth and decreasing degree of partial melting of the upper mantle. This presumably is an indication of a rapid uplifting and then waning magmatic hearth with gradually decreasing temperature, accompanied with down-cutting of the lithospheric faults. We call such a process “a reverse process of magma generation”. And the opposite process of the magmatic evolution of the East African rift in Kenya can be called “a positive process of magma generation”.     

Petrogenesis of 3.15 Ga old Banasandra komatiites from the Dharwar craton,India: Implications for early mantle heterogeneity

Spinifex-textured.magnesian(MgO 25 wt.%) komatiites from Mesoarchean Banasandra greenstone belt of the Sargur Group in the Dharwar craton,India were analysed for major and trace elements and~(147,146)Sm-~(143,142)Nd systematics to constrain age,petrogenesis and to understand the evolution of Archean mantle.Major and trace element ratios such as CaO/Al_2O_3.Al_2O_3/TiO_2,Gd/Yb,La/Nb and Nb/Y suggest aluminium undepleted to enriched compositional range for these komatiites.The depth of melting is estimated to be varying from 120 to 240 km and trace-element modelling indicates that the mantle source would have undergone multiple episodes of melting prior to the generation of magmas parental to these komatiites.Ten samples of these komatiites together with the published results of four samples from the same belt yield ~(147)Sm-~(143)Nd isochron age of ca.3.14 Ga with an initial ε_(Nd)(f) value of+3.5.High precision measurements of ~(142)Nd/~(144)Nd ratios were carried out for six komatiite samples along with standards AMES and La Jolla.All results are within uncertainties of the terrestrial samples.The absence of~(142)Nd/~(144)Nd anomaly indicates that the source of these komatiites formed after the extinction of ~(146)Sm,i.e.4.3 Ga ago.In order to evolve to the high ε_(Nd)(t) value of +3.5 by 3.14 Ga the time-integrated ratio of~(147)Sm/~(144)Nd should be 0.2178 at the minimum.This is higher than the ratios estimated,so far,for mantle during that time.These results indicate at least two events of mantle differentiation starting with the chondritic composition of the mantle.The first event occurred very early at ~4.53 Ga to create a global early depleted reservoir with superchondritic Sm/Nd ratio.The source of Isua greenstone rocks with positive ~(142)Nd anomaly was depleted during a second differentiation within the life time of ~(146)Sm,i.e.prior to 4.46 Ga.The source mantle of the Banasandra komatiite was a result of a differentiation event that occurred after the extinction of the ~(146)Sm,i.e.at 4.3 Ga and prior to 3.14 Ga.Banasandra komatiites therefore provide evidence for preservation of heterogeneities generated during mantle differentiation at4.3 Ga.     

Physical volcanology and geochemistry of Palaeoarchaean komatiite lava flows from the western Dharwar craton,southern India: implications for Archaean mantle evolution and crustal growth

ABSTRACT

Palaeoarchaean (3.38–3.35 Ga) komatiites from the Jayachamaraja Pura (J.C. Pura) and Banasandra greenstone belts of the western Dharwar craton, southern India were erupted as submarine lava flows. These high-temperature (1450–1550°C), low-viscosity lavas produced thick, massive, polygonal jointed sheet flows with sporadic flow top breccias. Thick olivine cumulate zones within differentiated komatiites suggest channel/conduit facies. Compound, undifferentiated flow fields developed marginal-lobate thin flows with several spinifex-textured lobes. Individual lobes experienced two distinct vesiculation episodes and grew by inflation. Occasionally komatiite flows form pillows and quench fragmented hyaloclastites. J.C. Pura komatiite lavas represent massive coherent facies with minor channel facies, whilst the Bansandra komatiites correspond to compound flow fields interspersed with pillow facies. The komatiites are metamorphosed to greenschist facies and consist of serpentine-talc ± carbonate, actinolite–tremolite with remnants of primary olivine, chromite, and pyroxene. The majority of the studied samples are komatiites (22.46–42.41 wt.% MgO) whilst a few are komatiitic basalts (12.94–16.18 wt.% MgO) extending into basaltic (7.71 – 10.80 wt.% MgO) composition. The studied komatiites are Al-depleted Barberton type whilst komatiite basalts belong to the Al-undepleted Munro type. Trace element data suggest variable fractionation of garnet, olivine, pyroxene, and chromite. Incompatible element ratios (Nb/Th, Nb/U, Zr/Y Nb/Y) show that the komatiites were derived from heterogeneous sources ranging from depleted to primitive mantle. CaO/Al₂O₃ and (Gd/Yb)_N ratios show that the Al-depleted komatiite magmas were generated at great depth (350–400 km) by 40–50% partial melting of deep mantle with or without garnet (majorite?) in residue whilst komatiite basalts and basalts were generated at shallow depth in an ascending plume. The widespread Palaeoarchaean deep depleted mantle-derived komatiite volcanism and sub-contemporaneous TTG accretion implies a major earlier episode of mantle differentiation and crustal growth during ca. 3.6–3.8 Ga.