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11.
How late are K-feldspar megacrysts in granites? 总被引：1，自引：0，他引：1
Various petrologists have suggested that K-feldspar megacrysts grow in granites that are extensively crystallized, even at subsolidus conditions. However, experimental evidence indicates that, though K-feldspar nucleates relatively late in the crystallization history, abundant liquid is available for development of large crystals. A great deal of evidence, involving many different factors, favours a magmatic/phenocrystic origin for K-feldspar megacrysts in granites, namely simple twinning, oscillatory zoning, euhedral plagioclase inclusions, and concentric, crystallographically controlled arrangements of inclusions. In addition, abundant evidence has been presented of (1) mechanical accumulation of K-feldspar megacrysts in granites, (2) alignment of megacrysts and megacryst concentrations in magmatic flow foliations, (3) involvement of megacrysts in zones of magma mixing in granite plutons, and (4) occurrence of megacrysts in some volcanic rocks, implying that the megacrysts were suspended in enough liquid to be moved without fracturing or plastic deformation. Detailed trace element and isotopic data also indicate that megacrysts can move between coexisting felsic and more mafic magmas. Irregular overgrowths on megacrysts are consistent with continued magmatic growth after euhedral megacrystic growth ceased, the overgrowths being impeded by simultaneously crystallizing quartz and feldspar grains. 相似文献
12.
Youjuan Li Dewen Zheng Ying Wu Ying Wang Huaiyu He Jianzhang Pang Yizhou Wang Jingxing Yu 《Geostandards and Geoanalytical Research》2017,41(3):359-365
In this study (U‐Th)/He dating of the Penglai zircons, which occur as abundant megacrysts in Neogene alkaline basalts in northern Hainan Province, south‐eastern China, was undertaken. A weighted mean age of 4.06 ± 0.35 Ma (2s) with a mean square weighted deviation (MSWD) of 1.79 was obtained from eighteen fragments of four zircon megacrysts using single‐crystal laser fusion He determinations and the U‐Th isotope dilution (ID) method. The (U‐Th)/He ages are consistent, homogeneous and systematically slightly younger than the preferred 206Pb/238U age of 4.4 ± 0.1 Ma (95% confidence interval) determined by ID‐TIMS and subsequently published U‐Pb results. The U‐Pb isotopic system in zircon has a high closure temperature of ~ 900 °C, and the preferred U‐Pb age may record both the time since eruption and the zircon residence time in the magma chamber. In contrast, the closure temperature of the zircon (U‐Th)/He system is ~ 190 °C and the zircon megacrysts were brought quickly to the surface by the host basaltic magma. Thus, the (U‐Th)/He age represents the timing of the eruption. Based on the unlimited quantity, large grain size, mostly weak broad zoning, rapid cooling and homogenous (U‐Th)/He ages, we consider the Penglai zircons suitable for use as a reference material in (U‐Th)/He isotope geochronology. 相似文献
13.
安徽铜陵地区中生代幔源岩浆底侵作用——来自矿物巨晶和岩石包体的证据 总被引：13，自引：2，他引：13
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
14.
Hf Isotope Systematics of Kimberlites and their Megacrysts: New Constraints on their Source Regions 总被引：10，自引：5，他引：10
NOWELL G. M.; PEARSON D. G.; BELL D. R.; CARLSON R. W.; SMITH C. B.; KEMPTON P. D.; NOBLE S. R. 《Journal of Petrology》2004,45(8):1583-1612
Kimberlites from Southern Africa, along with their low-Cr megacrysts,have unusual Hf–Nd isotopic characteristics. Group I andTransitional kimberlites define arrays trending oblique to,and well below, the Nd–Hf isotope ‘mantle array’,defined by oceanic basalts, i.e. they have negative 相似文献
15.
16.
K-feldspar megacrysts (Kfm) are used to investigate the magmaticevolution of the 7 Ma Monte Capanne (MC) monzogranite (Elba,Italy). Dissolution and regrowth of Kfm during magma mixingor mingling events produce indented resorption surfaces associatedwith high Ba contents. Diffusion calculations demonstrate thatKfm chemical zoning is primary. Core-to-rim variations in Ba,Rb, Sr, Li and P support magma mixing (i.e. high Ba and P andlow Rb/Sr at rims), but more complex variations require othermechanisms. In particular, we show that disequilibrium growth(related to variations in diffusion rates in the melt) may haveoccurred as a result of thermal disturbance following influxof mafic magma in the magma chamber. Initial 87Sr/86Sr ratios(ISr) (obtained by microdrilling) decrease from core to rim.Inner core analyses define a mixing trend extending towardsa high ISr–Rb/Sr melt component, whereas the outer coresand rims display a more restricted range of ISr, but a largerrange of Rb/Sr. Lower ISr at the rim of one megacryst suggestsmixing with high-K calc-alkaline mantle-derived volcanics ofsimilar age on Capraia. Trace element and isotopic profilessuggest (1) early megacryst growth in magmas contaminated bycrust and refreshed by high ISr silicic melts (as seen in theinner cores) and (2) later recharge with mafic magmas (as seenin the outer cores) followed by (3) crystal fractionation, withpossible interaction with hydrothermal fluids (as seen in therim). The model is compatible with the field occurrence of maficenclaves and xenoliths. KEY WORDS: Elba; monzogranite; K-feldspar megacrysts; zoning; magma mixing; trace element; Sr isotopes; petrogenesis 相似文献
17.
Helium isotope studies of the mantle xenoliths and megacrysts from the Cenozoic basalts in the eastern China 总被引：9，自引：2，他引：7
Helium isotope compositions of the mantle xenoliths and megacrysts in the Cenozoic basalts in the eastern China were measured.
The samples were collected from Ludao of Heilongjiang, Huinan and Jiaohe of Jilin, Kuandian of Liaoning, Hannuoba of Hebei,
Nüshan of Anhui, Dingan of Hainan. The3He/4He ratios of the mantle xenoliths and megacrysts from the most areas were about 1 × 10-5, and were similar to those of the MORB, thus reflecting the characteristics of the MORB-typed depleted mantle. The3He/4He ratios of the mantle xenoliths from Jiaohe were 4.8×10-6 and the3He/4He ratios of xenoliths from Hannuoba vary from 0.15× 10-6 to 7.4 ×10-6, obviously lower than those of the MORB, and even lower than the atmospheric helium isotope ratios, indicating that the continental
mantle was strongly replaced in Jiaohe and Hannuoba areas. The helium isotope compositions of the mantle xenoliths and megacrysts
in the same region vary in a very wide range. It is inferred that the mantle xenoliths and megacrysts were from different
parts of the continental mantle. There were not necessary origin relations between the mantle xenoliths, megacrysts and their
host basalts. An extremely high3He/4He ratio of garnet megacryst from Hannuoba, Hebei was found. 相似文献
18.
Mafic alkaline lavas from the Venetian Volcanic Province (NE Italy) contain orange–brown zircon megacrysts up to 15 mm long, subhedral to subrounded and showing equant morphology, with width-to-length ratios of 1:2–1:2.5. U–Pb ages of zircon (51.1 ± 1.5 to 30.5 ± 0.51 Ma) fit the stratigraphic age of the host lava (Middle Eocene and Oligocene) and their oxygen isotope composition (δ18O = 5.31–5.51‰) is similar to that of zircon formed in the upper mantle. Cathodoluminescence images and crystal chemical features, e.g. depletion of incompatible elements such as REE, Y, U and Th at constant Hf content, indicate that centre-to-edge zircon zoning is not consistent with evolution of the melt by fractional crystallization. All the above features, together with the fact that zircon and host basalts are coeval, indicate that the studied Zr megacrysts crystallised from a primitive alkaline mafic magma, which later evolved to the less alkaline host magma. 相似文献
19.
Origin of lamprophyres by the mixing of basic and alkaline melts in magma chamber in Beiya area, western Yunnan, China 总被引：6，自引：0，他引：6
Lamprophyres consisting mainly of diopside, phlogopite and K-feldspar formed in the early Tertiary around 60 Ma in the Beiya area and are characterized by low SiO2 ± 46–50 wt.%), Rb (31–45 ppm) and Sr (225–262 ppm), high Al2O3, (11.2–13.1 wt.%), CaO (8.0–8.7 wt.%), MgO (11.5–12.1 wt.%), K2O(4.9–5.5 wt.%), TiO2 (2.9–3.3 wt.%) and REE (174–177 ppm), and compatible elements (e.g. Sc, Cr and Ni) and HSF elements (e.g. Th, U, Zr, Nb, Ta, Ti and Y), and low 143Nd/144Nd 0.512372–0.512536, middle 87Sr/86Sr 0.707322–0.707395, middle 206Pb/204Pb 18.50–18.59, 207Pb/204Pb 15.60–15.65 and 208Pb/204Pb 38.75–38.8. These rocks developed peculiar quartz megacrysts with poly-layer reaction zones, melt inclusions, and partial melted K-feldspar and plagioclase inclusions, and plastic shapes. Important features of these rocks include: (1) hybrid composition of elements, (2) abrupt increase of SiO2 content of the melt, recorded by zoned diopside, (3) development of sanidine and aegirine-augite reaction zones, (4) alkaline melt and partial melted K-feldspar and plagioclase inclusions, (5) deformed quartz inclusions associated with quartz megacrysts, (6) the presence of quartz megacrysts in plastic shape with their parent melts, (7) the occurrence of olivine, high-MgO ilmenite and spinel inclusions within earlier formed diopside, phlogopite and magnetite. Median 87Sr/86Sr values between Tertiary alkaline porphyries in the Beiya area and the western Yunnan and Tertiary basalt in the western Yunnan indicate that the Beiya lamprophyre melts were derivative and resulted from the mixing between basic melts that were related to the partial melting of phenocrysts of spinel iherzolite from a mantle source. The alkaline melts originated from partial melting along the Jinshajiang subduction ductile shear zone at the contact between the buried Palaeo-Tethyan oceanic lithosphere and the upper mantle lithosphere. The alkaline melts are composed of 65% sanidine (Or70Ab28An2) and 35% SiO2. The melt mixing occurred in magma chambers in the middle-shallow crust at 8–10 km before the derivative lamprophyre melts intruded into the shallow cover in Beiya area. This mixing of basic and alkaline melts might represent a general process for the formation of lamprophyre in the western Yunnan. 相似文献
20.
李有柱 《地球科学与环境学报》1993,(4)
安徽女山新生代碧玄岩含有丰富的高压巨晶和幔源包体。地球化学数据和杂质元素资料表明,女山碧玄岩是原生的地幔二辉橄榄岩包体局部熔融的产物。局部熔融作用发生在1.7～2.15GPa和1350℃～1450℃的条件下。熔出碧玄岩熔浆的数量约为5％～10％体积。 相似文献