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題 名 | Alkali Syenites and Associated Rocks in Chintan, Northern Taiwan: A Reappraisal=再論臺灣北部青潭鹼性正長岩及其共存岩石 |
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作 者 | 陳正宏; | 書刊名 | 中國地質學會會刊 |
卷 期 | 41:3 1998.08[民87.08] |
頁 次 | 頁337-358 |
分類號 | 358.42 |
關鍵詞 | 鹼性正長岩; 岩脈和岩床; 即地岩漿分化作用; 臺灣北部; Alkali syenite; Dike and sill; In-situ magma differentiation; Northern Taiwan; |
語 文 | 英文(English) |
中文摘要 | 臺灣光復前日籍學者市村毅(Ichimura,1929,1943)所稱之青潭岩脈和岩床產狀的 鹼性正長岩有再檢討的必要, 肇因於其內所含的鹼性礦物並不明確,例如其所提及之鹼性 鐵鎂礦物 " 霓石 " 或 " 霓輝石 " 以及 " 棕閃石 ", 於電子探針之詳細偵測下確定並不 存在,實為 " 鈦次透輝石 " 以及 " 鈦角閃石 "。 由於此等岩石中之鹼性礦物僅有鹼性長 石(為條紋長石或反條紋長石形式),基於其相對含量之多寡及礦物顆粒之大小,以往所稱 之 " 鹼性正長岩 " 似應改稱為 " 微正長岩 ",而原稱之 " 優白色條帶 " 可改稱為 " 鹼 性長石正長岩 " 較為恰當。 從岩脈和岩床的正長岩類所分離出的鹼性長石以雷射氬氬法測 定所得的年斡各自為 11.2 ± 0.5Ma 及 21.2 ± 0.9Ma。 在岩脈中,此等正長岩類與方沸 石粗粒玄武岩和其邊緣玄武岩關係密切,各種不同岩石成因可視為岩脈侵入; 後產生即地礦 物分離作用之結果,以方沸石粗粒玄武岩代表沈聚物,正長岩類代表殘留熔體。無論從各岩 類之稀土元素分佈變化、鍶和釹同位素比值、;主要和微量元素含量之變化,均能支持這種 即地岩漿分化作用的模式,這也、和全球性與岩脈和岩床侵入有關的正長岩類的主要成因相 仿。 |
英文摘要 | The diked and silled alkali syenites in Chintan, northern Taiwan (Ichimura, 1929, 1943) have been re-studied due mainly to some ambiguities in defining their alkali constituent minerals. For instance, the alkali pyroxene ( "aegirine" or "aegirine-augite") and iron-rich alkali amphibole ("barkevikite" ) described in the above-mentioned literatures either are absent or should be renamed as revealed from a large number of electron microprobe data. The principal mafic minerals in these rocks are actually Tisalite and kaersutite. Further, because alkali feldspar (in the perthite/antiperthite form) is the only felsic mineral present in these rocks, the "microsyenite" and "alkali feldspar syenite", based on the relative proportion of the alkali feldspars, are proposed to replace, respectively, the old term "alkali syenite" and its "leucocratic bands" although the former is more dioritic geochemically. Alkali feldspars separated from the syenitic rocks in the dike and the sill are dated to be 11.2 ± 0.5 Ma and 21.2 ± 0.9 Ma respectively by using the Ar-Ar method with a focused laser source. These syenitic rocks occur in close association with teschenite in the dike in which a marginal basalt is developed. Petrogenetically, different rock types in the dike can be explained by mineral segregation from a common magma source at various temperature conditions after intrusion. Such an in-situ magma differentiation model, widely accepted to account for the occurrence of syenitic rocks in the form of a sill or dike, is not only supported by the fact that all these rocks show similar REE distribution patterns and Sr- and Nd-isotopic ratios, but also consistent with the linear trends in major and trace elements among rock types. |
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