Geochronology and petrogenesis of Pan-African, syn-tectonic, S-type and post-tectonic A-type granite (Namibia): products of melting of crustal sources, fractional crystallization and wall rock entrainment
Geochronology and petrogenesis of Pan-African, syn-tectonic, S-type and post-tectonic A-type granite (Namibia): products of melting of crustal sources, fractional crystallization and wall rock entrainment
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DOI：
10.1016/s0024-4937(99)00059-6
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发表时间：
2000-02
期刊：
影响因子：
3.5
通讯作者：
Stefan Jung;S. Hoernes;K. Mezger
中科院分区：
文献类型：
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作者：
Stefan Jung;S. Hoernes;K. Mezger
纳米比亚中部达马拉造山带Oetmoed花岗岩-杂岩杂岩杂岩（OGMC）主要由~ 526 ~ ~ 516 Ma含石榴石和堇青石的花岗岩和次要的~ 488 ~ ~ 494 Ma含角闪石和钛矿的花岗岩组成，呈平面片状和脉状。此外，一个稍微拉长的花岗岩体出现在复合体的中心。含石榴石和堇青石的花岗岩具有s型花岗岩的主元素和微量元素特征，而含角闪石和钛矿的花岗岩具有较高的HFSE和REE含量，与a型花岗岩相似。含石榴石和堇青石的花岗岩含有大量的弹性捕虏体，而含角闪石和钛矿的花岗岩则没有捕虏体。围岩为堇青石-硅线石-钾长石-石榴石变质沉积岩和混辉岩。s型花岗岩中富含堇青石和石榴石的捕虏体并不代表原生岩，它们的贫化化学成分最好的解释是合并的乡村岩石的不同程度的部分熔融。在含石榴石和堇青石的花岗岩中，大多数化学变化可以用斜长石、黑云母和附属相（主要是独居石和锆石）的分离结晶过程来解释。主要元素和微量元素数据以及高δ18O值表明，含石榴石和堇青石的花岗岩中演化最少的成员来自于800°C左右的变长岩。含角闪石和钛矿花岗岩具有较高的CaO和Na2O含量，较低的sio2含量和较低的Rb/Sr比值以及较低的δ18O值，表明它们更可能是在900℃以上的温度下由非泥质源（偏花岗岩？）的部分熔融形成的。降低TiO2， Na2O, FeOtot。在含角闪石和含钛花岗岩中，总REE、MgO、CaO含量增加，al2o3和K2O含量增加，表明角闪石和钛矿分选为主。含石榴石和堇青石的花岗岩与含角闪石和钛矿的花岗岩组成不同，是由不同的源岩（变长岩而非类变长岩）和不同的熔融温度造成的。此外，一些样品中明显的乡村岩石夹带作用在含石榴石和堇青石花岗岩的岩石形成过程中起主要作用，而在含角闪石和钛矿花岗岩的演化过程中起不重要作用。这种接近推断的变质作用高峰的热长英质岩浆的侵入，可能在一定程度上造成了相对低压下的高温变质作用和乡村岩石的深熔作用。
The Oetmoed Granite–Migmatite Complex (OGMC), Central Damara Orogen, Namibia, consists mainly of ∼526 to ∼516 Ma garnet- and cordierite-bearing granite and subordinate ∼488 to ∼494 Ma hornblende- and titanite-bearing granite in the form of planar sheets and dykes. Additionally, a slightly elongated granite body occurs in the center of the complex. The garnet- and cordierite-bearing granite has major- and trace-element characteristics of S-type granite but the hornblende- and titanite-bearing granite has higher HFSE and REE contents similar to A-type granite. Whereas the garnet- and cordierite-bearing granite contains numerous restitic xenoliths, the hornblende- and titanite-bearing granite is xenolith-free. The country rocks are cordierite–sillimanite–K-feldspar–garnet-bearing metasedimentary rocks and migmatite. Cordierite- and garnet-rich xenoliths in the S-type granite do not represent primary restite, their depleted chemical composition is best explained by varying and large degrees of partial melting of incorporated country rocks. Most chemical variations among the garnet- and cordierite-bearing granite can be explained by processes linked with fractional crystallization of plagioclase, biotite and accessory phases, mostly monazite and zircon. Major and trace element data and high δ18O values suggest that the least evolved members of the garnet- and cordierite-bearing granite were derived from metapelitic rocks at ca. 800°C as inferred from monazite and apatite dissolution thermometry. Higher CaO and Na2O but lower SiO2contents and lower Rb/Sr ratios as well as lower δ18O values of the hornblende- and titanite-bearing granite suggest that they are more likely generated by partial melting of non-pelitic sources (metagranitoids?) at temperatures in excess of 900°C. Decreasing TiO2, Na2O, FeOtot., MgO, CaO, total REE content but increasing Al2O3and K2O indicate fractionation of mainly hornblende and titanite in the case of the hornblende- and titanite-bearing granite. The differing compositions of the garnet- and cordierite-bearing granite and the hornblende- and titanite-bearing granite are attributed to different source rocks (metapelite instead of metagranitoid) and different temperatures during melting as inferred from accessory phase dissolution thermometry. Furthermore, significant entrainment of country rock in some samples played a major role during petrogenesis of the garnet- and cordierite-bearing granite but was not important during the evolution of the hornblende- and titanite-bearing granite. Intrusion of such hot, felsic magmas close to the inferred peak of metamorphism has probably caused, in part, the high temperature metamorphism and anatexis of the country rocks at relatively low pressures.