塔里木盆地库鲁克塔格地区二叠纪末-中三叠世基性岩床的发现及其地质意义

新疆塔里木盆地东北缘库鲁克塔格地区的中寒武统莫合尔山组沉积地层中发现三条形成于二叠纪末到中三叠世的基性岩床。三条岩床沿沉积岩层面顺层侵入,按照从下到上的顺序,分别为辉绿岩床(厚3m)、辉长岩床(厚20m)和粗玄岩床(厚2.5m)。三条岩床的地球化学特征相似,稀土含量高(∑REE=210×10^-6~297×10^-6),轻稀土元素富集((La/Yb)_N =13.54~17.96),Eu负异常不明显(δEu=0.97~1.21),Ce稍具正异常(δCe=1.11~1.25)。大离子亲石元素(Rb、Ba、Th、Sr)富集,Sr具明显正异常; Ta、Nb和Hf具负异常。低氧化度(0.13~0.40)。三条岩床为同源岩浆活动的产物,岩浆源于上地幔或下地壳,但有下陆壳成分混染。三个样品的MgO含量在4%~6%之间,Mg^#值在37.6~55.6之间,固结指数SI值位于18.7~29.3之间,反映原始的岩浆经历了比较明显的分异作用。粗玄岩床K-Ar年龄为255.2±7.3Ma,辉绿岩床K-Ar年龄为242.8±5.7Ma,辉长岩床Ar-Ar年龄为229.7±3.7Ma。本次岩浆侵入事件发生在南天山洋闭合之后,构造背景为造山后碰撞伸展环境。     

Geochemical characteristics of lavas from Broken Ridge, the Naturaliste Plateau and southernmost Kerguelen Plateau: Cretaceous plateau volcanism in the southeast Indian Ocean

Lavas from several major bathymetric highs in the eastern Indian Ocean that are likely to have formed as Early to Middle Cretaceous manifestations of the Kerguelen hotspot are predominantly tholeiitic; so too are glass shards from Eocene to Paleocene volcanic ash layers on Broken Ridge, which are believed to have come from eruptions on the Ninetyeast Ridge. The early dominance of tholeiitic compositions contrasts with the more recent intraplate, alkalic volcanism of the Kerguelen Archipelago. Isotopic and incompatible-element ratios of the plateau lavas are distinct from those of Indian mid-ocean ridge basalts; their Nd, Sr, ²⁰⁷Pb/²⁰⁴Pb and isotopic ratios overlap with but cover a much wider range than measured for more recent oceanic products of the Kerguelen hotspot (including the Ninetyeast Ridge) or, indeed, oceanic lavas from any other hotspot in the world. Samples from the Naturaliste Plateau and ODP Site 738 on the southern tip of the Kerguelen Plateau are particularly noteworthy, with ε_Nd(T) = − 13 to −7, (⁸⁷Sr/⁸⁶Sr)_T=0.7090 to 0.7130 and high ²⁰⁷Pb/²⁰⁴Pb relative to ²⁰⁶Pb/²⁰⁴Pb. In addition, the low-ε_Nd(T) Naturaliste Plateau samples are elevated in SiO₂ (> 54 wt%). In contrast to “DUPAL” oceanic islands such as the Kerguelen Archipelago, Pitcairn and Tristan da Cunha, the plateau lavas with extreme isotopic characteristics also have relative depletions in Nb and Ta (e.g., Th/Ta, La Nb > primitive mantle values); the lowest ε_Nd(T) and highest Th/Ta and La Nb values occur at sites located closest to rifted continental margins. Accepting a Kerguelen plume origin for the plateau lavas, these characteristics probably reflect the shallow-level incorporation of continental lithosphere in either the head of the early Kerguelen plume or in plume-derived magmas, and suggest that the influence of such material diminished after the period of plateau construction. Contamination of asthenosphere with the type of material affecting Naturaliste Plateau and Site 738 magmatism appears unlikely to be the cause of low-²⁰⁶Pb/⁰⁴Pb Indian mid-ocean ridge basalts. Finally, because isotopic data for the plateaus do not cluster or form converging arrays in isotope-ratio plots, they provide no evidence for either a quickly evolving, positive ε_Nd, relatively high-²⁰⁶Pb/²⁰⁴Pb plume composition, or a plume source dominated by mantle with ε_Nd of −3 to 0.     

Tholeiitic and alkali basalts from the Mid-Atlantic Ridge at 43 ° N

Tholeiitic basalts dredged from the Mid-Atlantic Ridge (MAR) axis at 43 ° N are enriched in incompatible trace elements compared to the ‘ normal’ incompatible element depleted tholeiites found from 49 ° N to 59 ° N and south of 33 ° N on the MAR. The most primitive 43 ° N glasses have MgO/FeO*= 1.2 and coexist with olivine (Fo_90–91) and chrome-rich spinel. The tholeiitic basalts from the MAR 43 ° N are distinct from the strongly incompatible trace element depleted tholeiities found elsewhere in the Atlantic, and have trace element features typical of island tholeiities and MAR axis tholeiites from 45 ° N. Petrographic, major, and compatible trace element trends of the axial valley tholeiites at 43 ° N are consistent with shallow-level fractionation; in particular, evolution from primitive liquids with forsteritic olivine plus chrome spinel as liquidus phases to fractionated liquids with plagioclase plus clinopyroxene as major crystallizing phases. However, each dredge haul has distinctive incompatible trace element abundances. These trace element characteristics require a hetrogeneous mantle or complex processes such as open system fractional crystallization and magma mixing. Alkali basalts (～5% normative nepheline) were dredged from a prominent fracture zone at 43 ° N. Typical of alkali basalts they are strongly enriched (compared to tholeiites) in incompatible elements. Their highly fractionated rare-earth element (REE) abundances require residual garnet during partial melting. The 43 ° N tholeiites and alkali basalts could be derived from a garnet peridotite source with REE contents equal to 2 × chondrites by ～5% and 1% melting, respectively. Alternatively, they could be derived from a moderately light REE enriched source by ～25% and 9.5% melting, respectively.     

Modelling phase–assemblage diagrams for magnesian metapelites in the system K2O–FeO–MgO–Al2O3–SiO2–H2O: geodynamic consequences for the Monte Rosa nappe, Western Alps

Magnesian metamorphic rocks with metapelitic mineral assemblage and composition are of great interest in metamorphic petrology for their ability to constrain P–T conditions in terranes where metamorphism is not easily visible. Phase–assemblage diagrams for natural and model magnesian metapelites in the system KFMASH are presented to document how phase relationships respond to water activity, bulk composition, pressure and temperature. The phase assemblages displayed on these phase diagrams are consistent with natural mineral assemblages occurring in magnesian metapelites. It is shown that the equilibrium assemblages at high pressure conditions are very sensitive to a(H₂O). Specifically, the appearance of the characteristic HP assemblage chloritoid–talc–phengite–quartz (with excess H₂O) in the magnesian metapelites of the Monte Rosa nappe (Western Alps) is due to the reduction of a(H₂O). Furthermore, the mineral assemblages are determined by the whole-rock FeO/(FeO+MgO) ratio and effective Al content X _A as well as P and T. The predicted mineral associations for the low- and high-X _A model bulk compositions of magnesian metapelites at high pressure are not dependent on the X _A variations as they show a similar sequence of mineral assemblages. Above 20 kbar, the prograde sequence of assemblages associated with phengite (with excess SiO₂ and H₂O) for low- and high-X _A bulk compositions of magnesian metapelites is: carpholite–chlorite → chlorite–chloritoid → chloritoid–talc → chloritoid–talc–kyanite → talc–garnet–kyanite → garnet–kyanite ± biotite. At low to medium P–T conditions, a low-X _A stabilises the phengite-bearing assemblages associated with chlorite, chlorite + K-feldspar and chlorite + biotite while a high-X _A results in the chlorite–phengite bearing assemblages associated with pyrophyllite, andalusite, kyanite and carpholite. A high-X _A magnesian metapelite with nearly iron-free content stabilises the talc–kyanite–phengite assemblage at moderate to high P–T conditions. Taking into account the effective bulk composition and a(H₂O) involved in the metamorphic history, the phase–assemblage diagrams presented here may be applied to all magnesian metapelites that have compositions within the system KFMASH and therefore may contribute to gaining insights into the metamorphic evolution of terranes. As an example, the magnesian metapelites of the Monte Rosa nappe have been investigated, and an exhumation path with P–T conditions for the western roof of the Monte Rosa nappe has been derived for the first time. The exhumation shows first a near-isothermal decompression from the Alpine eclogite peak conditions around 24 kbar and 505°C down to approximately 8 kbar and 475°C followed by a second decompression with concomitant cooling.M. Frey: deceased     

东南沿海地区第四纪大陆岩石圈地幔的特征

东南沿海地区新生代玄武岩中的橄榄岩包体来自年轻的大陆岩石圈地幔 ,该岩石圈地幔在岩石学、矿物组成、痕量元素以及Sr Nd同位素组成等各方面具有很大差异。这些差异反映了它们来自不同的地幔过程。南海张开与地幔热柱有关 ,南海扩张后第四纪形成的火山岩携至地表的包体更多保留了地幔热柱的信息。橄榄岩包体的矿物成分与深海橄榄岩类似 ,相对贫Opx而富Ol;在痕量元素上 ,表现为强不相容元素的富集 ,其配分模式类似于其寄主岩 ;Nd同位素强烈亏损 ,显示出比MORB源区更亏损的特征。大陆岩石圈地幔经历了来自地幔深处的贫SiO2 熔体的进一步改造。     

The Ronda high temperature peridotite: Geochemistry and petrogenesis

The Ronda peridotite in southern Spain is a large (~300 km²) exposure of upper mantle which provides direct information about mantle processes on a scale much larger than that provided by mantle xenoliths in basalt. Ronda peridotites range from harzburgite to lherzolite, and vary considerably in major element content, e.g., Al₂O₃ from 0.9 to 4.8%, and trace element abundances, e.g., Sr, Zr and La abundances vary by factors of 20 to 40. These compositional variations are systematic and correlate with (pyroxene + garnet)/olivine ratios and olivine compositions. The data are consistent with formation of residual peridotites by variable degrees of melting (~0 to 30%) of a compositionally homogeneous peridotite. None of the peridotites have geochemical characteristics of residues formed by extensive (?5%) fractional melting and the data can be explained by equilibrium (batch) melting, possibly with incomplete melt segregation in some samples. Based on compositional differences between Ronda peridotites, the segregated melts were picritic (12–22% MgO) with relative rare earth element abundances similar to mid-ocean ridge basalt (MORB). Prior to the melting event the Ronda peridotite body was a suitable source for MORB. The compositional characteristics of Ronda peridotites are consistent with diapiric rise of a fertile mantle peridotite with relatively small degrees of melting near the diapir-wall rock interface yielding residues of garnet iherzolite, and larger degrees of melting in the diapir interior yielding residues of garnet-free peridotite. Subsequently these residual rocks were recrystallized at sub-solidus conditions (Obata, 1980), and emplaced in the crust by thrusting (Lundeen, 1978).     

Distribution of trace elements between garnet megacrysts and host volcanic liquids of kimberlitic to rhyolitic composition

Abundances of rare earth elements, Hf, Sc, Co, Cr and Th in garnet megacrysts and their volcanic hosts or matrices are used to estimate garnet/liquid partition coefficients for these elements. Samples include pyropes from kimberlite and highly alkalic basalts, almandines from basalt andesite, dacite, rhyodacite and rhyolite and a spessartine-almandine from alaskite. The pyrope/host partition coefficients are fairly uniform and agree with experimental data within a factor of 2. The almandine/matrix data show more scatter (due in part to impurities in the garnet separates) but the partition coefficients tend to increase with increasing $\text{Si}\text{O}$ ratio of the matrix. The almandine/matrix partition coefficients are up to a factor of 10 higher than the pyrope/host partition coefficients. The spessartine-almandine is strongly enriched in heavy rare earths (~ 5000 times chondrites), Y, Sc and Co. The wide variation in garnet/liquid partition coefficients from kimberlites to rhyolites cannot be explained as an effect of temperature and we conclude that a major factor is the composition of the melt from which the garnet crystallized.     

Calbuco Volcano and minor eruptive centers distributed along the Liquiñe-Ofqui Fault Zone,Chile (41°–42° S): contrasting origin of andesitic and basaltic magma in the Southern Volcanic Zone of the Andes

Calbuco volcano is a Late Pleistocene-Holocene composite stratovolcano located at 41°20 S, in the southern region of the Southern Volcanic Zone of the Andes (SSVZ; 37°–46° S). In contrast to basalt and basaltic andesite, which are the dominant lava types on the volcanic front from 37° to 42° S, Calbuco lavas are porphyritic andesites which contain a wide variety of crustal xenoliths. They have SiO₂ contents in the 55–60% range, and have comparatively low K₂O, Rb, Ba, Th and LREF abundances relative to other SSVZ centers. Incompatible element abundance ratios are similar to those of most SSVZ volcanics, but ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd are respectively higher and lower than those of adjacent volcanic centers. Basalts from nearby Osorno stratovolcano, 25 km to the northeast, are similar to other basaltic SSVZ volcanoes. However, basalts from several minor eruptive centers (MEC), located east of Calbuco and Osorno volcano along the Liquiñe-Ofqui fault zone (LOFZ), are enriched in Ba, Nb, Th and LREE, and have higher La/Yb and lower Ba/La, K/La and Rb/La. ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd in MEC basalts are respectively lower and higher than those of Osorno and Calbuco lavas. We suggest that MEC basalts were produced by lower extents of mantle melting than basalts from Osorno and other SSVZ stratovolcanoes, probably as a result of lower water content in the source of MEC basalts. Calbuco andesites formed from basaltic parents similar to Osorno basalts, by moderate pressure crystallization of a hornblende-bearing assemblage accompanied by crustal assimilation. Hornblende stability in the Calbuco andesites was promoted by the assimilation of hydrous metasedimentary crustal rocks, which are also an appropriate endmember for isotopic trends, together with magma storage at mid-crustal depths. The unique characteristics of Calbuco volcano, i.e. the stability of hornblende at andesitic SiO₂ contents, low ¹⁴³Nd/¹⁴⁴Nd and high ⁸⁷Sr/⁸⁶Sr, and abundant crustal xenoliths, provide evidence for crustal assimilation that is not apparent at more northerly volcanoes in the SSVZ.     

The tholeiite to alkalic basalt transition at Haleakala Volcano,Maui, Hawaii

Previous studies of alkalic lavas erupted during the waning growth stages (<0.9 Ma to present) of Haleakala volcano identified systematic temporal changes in isotopic and incompatible element abundance ratios. These geochemical trends reflect a mantle mixing process with a systematic change in the proportions of mixing components. We studied lavas from a 250-m-thick stratigraphic sequence in Honomanu Gulch that includes the oldest (1.1 Ma) subaerial basalts exposed at Haleakaka. The lower 200 m of section is intercalated tholeiitic and alkalic basalt with similar isotopic (Sr, Nd, Pb) and incompatible element abundance ratios (e.g., Nb/La, La/Ce, La/Sr, Hf/Sm, Ti/Eu). These lava compositions are consistent with derivation of alkalic and tholeiitic basalt by partial melting of a compositionally homogeneous, clinopyroxene-rich, garnet lherzolite source. The intercalated tholeiitic and alkalic Honomanu lavas may reflect a process which tapped melts generated in different portions of a rising plume, and we infer that the tholeiitic lavas reflect a melting range of 10% to 15%, while the intercalated alkalic lavas reflect a range of 6.5% to 8% melting. However, within the uppermost 50 m of section. ⁸⁷Sr/⁸⁶Sr decreases from 0.70371 to 0.70328 as eruption age decreased from 0.97 Ma to 0.78 Ma. We infer that as lava compositions changed from intercalated tholeiitic and alkalic lavas to only alkalic lavas at 0.93 Ma, the mixing proportions of source components changed with a MORB-related mantle component becoming increasingly important as eruption age decreased.     

Ages,rare-earth element enrichment,and petrogenesis of tholeiitic and alkalic basalts from Kahoolawe Island,Hawaii

Kahoolawe Island, Hawaii (18×11 km), is a basaltic shield volcano with caldera-filling lavas, seven identified postshield vents, and at least two occurrences of apparent rejuvenated-stage eruptive. We examined 42 samples that represent all stages of Kahoolawe volcano stratigraphy for their petrography, whole-rock major-and trace-element contents, mineral compositions, and K–Ar ages. The two oldest shield samples have an average age of 1.34±0.08 Ma, and four postshield samples (3 are alkalic) average 1.15±0.03 Ma; ages of 1.08 and 0.99 Ma for two additional tholeiitic samples probably are minimum ages. Whole-rock major- and trace-element and mineral compositions of Kahoolawe shield and caldera-fill laves are generally similar to the lavas forming Kilauea and Mauna Loa tholeiitic shields, but in detail, Kahoolawe shield lavas have distinctive compositions. An unusual aspect of many postshield Ka-hoolawe lavas is anomalously high REE and Y abundances (up to 200 ppm La and 175 ppm Y) and negative Ce anomalies. These enrichments reflect surficial processes, where weathering and soil development promoted REE-Y transport at the weathering front. Major element abundances (MgO, 10–6 wt.%) for shield and caldera-fill basalts are consistent with fractionation of ol+px+pl in frequently replenished magma reservoirs. In general, tholeiitic basalts erupted from late vents are higher in SiO₂ than the shield lavas, and temporal differences in parental magma compositions are the likely explanation. Alkalic basalts that erupted from vents are comparable in composition to those at other Hawaiian volcanoes. Trace-element abundance ratios indicate that alkalic basalts represent either relatively lower degrees of melting of the shield source or a distinct source. Apparent rejuvenated-stage basalts (i.e., emplaced after substantial Kahoolawe erosion) are tholeiitic, unlike the rejuvenated-stages at other Hawaiian volcanoes (alkalic). Kahoolawe, like several other Hawaiian volcanoes, has intercalated tholeiitic and alkalic basalts in the postshield stage, but it is the only volcano that appears to have produced tholeiitic rejuvenated-stage lavas.