江西足洞和关西花岗岩的岩石学、稀土元素地球化学及成岩机制

PETROLOGY, REE GEOCHEMISTRY AND ROCK-FORMING MECHANISM OF THE ZUDONG AND GUANXI GRANITES, JIANGXI

通过对足洞和关西两个岩体形成地质背景、岩石学、岩石稀土元素地球化学、成岩时代、铅和氧同位素组成以及初始锶比值的研究,充分表明,足洞岩体由黑云母钾长花岗岩和白云母钾长-碱长花岗岩组成,是燕山中晚期(124Ma)形成的富HREE花岗岩体;关西黑云母碱长花岗岩则是燕山早期(170Ma)的富LREE花岗岩体。前者的特征相当于S型花岗岩,后者相当于兼有S型特征的Ⅰ型花岗岩。由于成岩的源岩性质、再生岩浆REE含量和配分以及自交代程度的不同,是造成这两个岩体所含重、轻稀土差别的根本原因,同时这种稀土内生地球化学的差异,也决定了这两个岩体风化壳内重、轻稀土次生富集成矿的继承性特征。

The Zudong and Guanxi intrusions occur in the post Caledonian Southern Jiangxi uplift area of the South China Caledonian fold belt, and have an intrusive contact with the Lower Jurassic volcanics. The Zudong intrusion is composed of biotite-K- feldspar granite and muscovite-K-feldspar-alkali-feldspar granite, the latter being the product of deuteric autometasomatic muscovitization of the former, without abrupt boundary in between (Fig. 1). The Guanxi intrusion mainly is biotile-alkali-feldspar granite. The Zudong intrusion is characteristically poor in Ca, high in Si and rich in K, Na and F, while the Guanxi intrusion is poor in Ca and F, high in Si and rich in K and Na (Table 4), and the two intrusions belong to the subalkalic granite series (Fig. 5). For the Zudong intrusion, in particular, the muscovite- K-feldspar-alkali-feldspar granite differs from the biotite-alkali- feldspar granite of the Guanxi intrusion by the wide development of autometasomatic albitization, muscovitization and fluorite - doverite mineralization. The average REE content of the Zudong intrusion is 264 × 10-6. When the biotite-K-feldspar granite was trans formed into the muscovite-K-feldspar-alkali-feldspar granite, the average LREE/ HREE ratio changed from 1 : 1.2 to 1 : 4, and the average Y/ E∑REE ratio increased from 35.78 to 54.5 %, which shows the Yttrium - rich feature of the muscovite-K-feldspar-alkali- feldspar granite (Table 5, Fig. 6), The REE contents of rock-forming minerals of the Zudong intrusion make up 14% of the whole-rock REE content, suggesting that REE of the intrusion are chiefly contributed by REE-minerals or REE-bearing minerals (such as monazite, xenotime, doverite, gadolinite, chernovite, fluorite and zircon). The average REE content of the Guanxi intrusion is 358.77 × 10-6, the average LREE/ HREE ratio being 2.4 and the average Y /∑REE ratio being only 18.1 %, show- ing that the intrusion is mainly rich in LREE.On the basis of the studies of the geological setting, petrochemistry and zircon population type of the two intrusions, combined with the determinations of their rock - forming ages, oxygen and lead isotopes and initial 87Sr/ 86Sr ratios, the authors have come to the conclusions that (1) the Zudong intrusion (represented by the biotite-K-feldspar granite) corresponds to the S-type granite or ilmenite-series granite, containing a mineral assemblage of quartz-microcline-oligoclase-albite-biotite (muscovite)-ilmenite-Mn-grossular, with an average modal corundite content of 1.45%, A12O3/ (Na2O+K2O+CaO) ratio of 1.10, relativelly high radiogenic lead content (Table 8, Fig. 7), average whole-rock δ18O value of +8.95%?(Table 7), and initial 87Sr/ 86Sr ratio of 0.7188; (2) the Guanxi intrusion corresponds to the I-type granite or magnetite-series granite with a feature of the S- type granite, and contains a mineral assemblage of quartz- perthite- horoblende-biotite-magnetite, with an average modal corundite content of 0.5%, A12O3/ (Na2O+K2O+CaO) ratio of 1.4, also a high radiogenic lead content average whole-rock δ18Q value of +8.82‰, and initial 87Sr/ 86Sr ratio of 0.7056. Therefore, it can be inferred that during the early and middle-late Yanshanian, the intermediate-basic igneous rocks of felsic raetaraorphics of the Precambrian basement were remelted (or anatexised) due to the increase of the local heat-flow value, and the resultant granitic magmas of different magmatic hearths upwardly intruded along fault zones, thus forming the passively emplaced Guanxi and Zudong intrusions of different ages (170Ma and 124Ma respectively). However, the difference in REE partitioning and REE mineralization of the Zudong and Guanxi intrusions was attribution to the differences in the original HREE and LREE contents of the regenerated magmas forming the two intrusions and thedeuteric autometasomatism.