Research on the trace element contents of granitic rocks

为明确韩国和日本东南部花岗质岩浆的分选模式，采用中子活化分析方法测定了花岗质岩石的微量元素丰度，特别是稀土元素（REE）丰度和Ba、Rb、Sr关系。韩国侏罗纪花岗质岩石的稀土元素模式与韩国白垩系花岗质岩石的稀土元素模式有很大不同。侏罗纪模式的[Eu] /[Eu*]比值为1.1 ~ 0.62,(La/Lu)cn比值为80 ~ 20，总REE含量为100 ~ 180 ppm，白垩纪模式的比值为1.1 ~ 0.04，总REE含量为38 ~ 2.4，总REE含量为100 ~ 240。我们使用瑞利分馏模型作为花岗岩熔体的结晶模型。由于模型计算与白垩系花岗质岩石的稀土元素数据拟合较好，认为韩国白垩系花岗质岩石主要以分级结晶分异。侏罗纪花岗质岩石的稀土元素模式表明，石榴石或锆石是老变质岩部分熔融的残留物（Tsusue et al., 1987a）。根据花岗岩体的分布、产状、年龄和化学矿物组成，在日本西南部已确定了4个花岗岩体省。日本西南部中生代和第三纪花岗质岩石的稀土元素模式与韩国白垩系花岗质岩石的稀土元素模式相似。日本西南花岗质岩石[Eu]/[Eu*]比值为1.03 ~ 0.01,(La/Lu)cn比值为28 ~ 1.2，总REE含量为110 ~ 730 ppm。采用瑞利分馏模型对花岗岩熔体的结晶过程进行了模拟。由于模型计算与观测到的花岗质岩石稀土元素模式拟合较好，认为日本西南部花岗质岩石主要以分级结晶分异（Tsusue et al., 1987b）。McCarthy和Robb（1978）认为，花岗岩浆在原位分馏结晶过程中，液态矿物学从斜长岩-石英-黑云母到斜长岩-石英-钾长石-黑云母的变化对Ba、Rb、Sr相互关系的丰度有显著影响。在斜长岩-石英-黑云母分馏过程中，Ba、Rb在连续的固体和熔体中富集，而Sr则贫化。钾长石作为固相的进入，导致Ba随Sr相继形成固体而被贫化，而Rb则继续富集。总的来说，来自日本西南部花岗质岩石的微量元素数据很好地拟合了该模型，尽管观测到的数据的散点比积云-积云间比例的简单变化所预测的散点要大一些——无论如何，这可能主要是由于原始熔体中Ba、Rb和Sr含量的差异以及同化效应。少

To clarify the mode of differentiation of granitic magmas of South Korea and Southeast Japan, a neutron activation analysis has been made to measure the trace element abundance, especially rare earth element (REE) abundance and Ba, Rb and Sr relationships of the granitic rocks. The REE patterns of the Jurassic granitic rocks in South Korea are quite different from the patterns of the Cretaceous granitic rocks in South Korea. Namely, the [Eu] /[Eu*] ratio ranges from 1.1 to 0.62, the (La/Lu)cn ratio changes from 80 to 20, and total REE content varies from 100 to 180 ppm for Jurassic patterns, while the same ratios and the total REE content range from 1.1 to 0.04, from 38 to 2.4, and from 100 to 240, respectively, for the Cretaceous patterns. We used the Rayleigh fractionation model as a crystallization model of the granitic melts. Since the model calculation fit the REE data for the Cretaceous granitic rocks very well, it is suggested that the cretaceous granitic rocks in South Korea we … More re mainly differentiated by fractional crystallization. The REE patterns for the Jurassic granitic rocks suggest that participation of garnet or zircon as residues of partial melting of old metamorphic rocks (Tsusue et al., 1987a).Four granite provinces have been recongnized in Southwest Japan on the basis of their distribution, mode of occurrence, age and chemical and mineralogical composition of granitic rocks. The REE patterns of the Mesozoic and Tertiary granitic rocks of Southwest Japan are somewhat similar to the REE patterns of the Cretaceous granitic rocks of South Korea. Namely, [Eu]/[Eu*] ratio ranges from 1.03 to 0.01, the (La/Lu)cn ratio changes from 28 to 1.2, and total REE content varies from 110 to 730 ppm for the granitic rocks of Southwest Japan. We employed the Rayleigh fractionation model to simulate the crystallization of granitic melts. Since the model calculation fit the observed REE patterns for the granitic rocks well, it is suggested that the granitic rocks of Southwest Japan were mainly differentiated by fractional crystallization (Tsusue et al., 1987b).As suggested by McCarthy and Robb (1978), a change in liquidus mineralogy form plagioclase-quartz-biotite to plagioclase-quartz-K-feldspar-biotite during the in situ fractional crystallization of a granitic magma has a marked effect on the abundance of an interrelationships between Ba, Rb and Sr. During plagioclase-quartz-biotite fractionation, Ba and Rb enrich in successive solid and melt, while Sr is depleted. The incoming of K-feldspar as a solid phase causes Ba to be depleted along with Sr insuccessively formed solid, While Rb continues to be enriched. In general, trace element data from the granitic rocks of Southwest Japan fit the model well, although the observed scatter of the data is somewhat greater than predicted by simple variation in cumulus-intercumulus proportions - possibly due, at any rate mainly to a difference in Ba, Rb and Sr contents of original melts and to the effect assimilation. Less