土耳其東南部蛇綠岩套中超基性岩之地球化學特性及其隱沒帶特徵

Guleman和Kizildağ兩蛇綠岩套出露於土耳其東南方，為晚白堊紀特提斯造山帶 (Tethys orogenic belt) 中殘留的新特提斯洋(Neoethyan ocean) 海洋岩石圈殘塊。前人分析此蛇綠岩套中岩漿岩的地球化學特性，認為形成於隱沒起始 (subduction initiation) 時的岩漿活動；然而超基性岩的鐵同位素研究結果則支持海洋地殼應形成於中洋脊 (mid-ocean ridge, MOR) 的環境，顯示此區蛇綠岩套在成因上仍存在許多爭議。本研究分析兩套蛇綠岩套中超基性岩的地球化學特性，包括岩象學、主量元素、微量元素、礦物化學、錸-鋨同位素和鉑族金屬元素組成，藉以探討此蛇綠岩套的岩石成因。透過岩象學觀察，Guleman 和Kizildağ的超基性岩均為方輝橄欖岩(harzburgite)，但在組織 (texture) 上略有不同：(1) Guleman的樣本多呈殘碎斑狀組織，較未受蛇紋岩化，其斜輝石晶型為半自形 - 他形，粒徑偏小 ( < 0.2mm)；尖晶石則屬於高鉻尖晶石。Kizildağ的樣本均為等粒狀組織，且受到十分嚴重的蛇紋岩化作用 (燒失量 ~ 11wt%)，當中的橄欖石多遭受嚴重的熱水換質而轉變成蛇紋石，僅存少數的殘晶保有原始橄欖石特性；斜輝石含量極少，在光學顯微鏡下不易辨認，多生長在直輝石外圍，可能是直輝石和岩漿反應的產物；尖晶石的鉻含量則較Guleman的尖晶石低，在光學顯微鏡平行偏光下透光度較高。上述岩象學證據指出兩地曾遭受不同程度的應力擠壓及後期熱水換質作用(hydrothermal alteration)。將Guleman和Kizildağ橄欖岩樣本與現今中洋脊的深海橄欖岩 (abyssal peridotite) 及Izu-Bonin Mariana 弧前之橄欖岩進行全岩及礦物主量元素對比發現：Guleman橄欖岩的虧損程度大於所有的深海橄欖岩，而Kizildağ橄欖岩則是和最虧損的深海橄欖岩具有相近的化學成分。進一步透過微量元素模擬計算樣本所經歷的部分熔融程度，本研究認為以玻安岩 (Boninite) 的源區和形成條件作為計算參數的模擬結果最符合測量到的化學組成，計算結果得Guleman橄欖岩為約經歷25~30%部分熔融的殘餘，Kizildağ橄欖岩則是18~21%部分熔融的殘餘。於微量元素結果上，Guleman和Kizildağ的樣本都有大離子半徑元素 (U, Th, Pb) 和輕稀土元素 (LREE) 的富集，且鉑族金屬元素中較不相容的Pd, Re都較原始上部地函富集，且Pd/Ir和Yb皆呈現負相關，顯示二者曾在隱沒帶經歷過交代換質作用 (metasomatism)。由於上述證據均和玻安岩的岩石成因十分吻合，故本研究認為此二區域之方輝橄欖岩為隱沒起始時玻安岩岩漿活動的殘餘。從錸鋨同位素結果來看，所有Guleman 和Kizildağ橄欖岩的187Os/188Os均小於球富集粒隕石質地函 (chondritic mantle, 0.1278)，從0.11679~0.12706不等；全岩Re含量和187Re/188Os則多小於上部地函平均值 ( ~ 0.4)，可得知Guleman和Kizildağ橄欖岩均保有原始的鋨同位素訊號。其模式年齡主要分成三群，分別為100~200Ma、400~700Ma和1594Ma，其中200Ma左右的年齡可對應到新特提斯洋張裂的事件 (~125Ma)，其餘前寒武紀的年齡可能記錄著當地新生地殼生成的岩漿事件，亦有可能反應上部地函鋨同位素的不均值性。

關鍵字

蛇綠岩；超基性岩；地球化學；鋨同位素地球化學；鉑族金屬元素地球化學；特提斯造山帶

並列摘要

Ophiolites from SE Turkey have been recognized as SSZ-type and interpreted as remnants of the Neotethyan oceanic lithosphere. In this study we provide constraints from geochemistry of peridotites in Kizildağ and Guleman ophiolites. The Guleman harzburgites are fresh with L.O.I. from 0.3~2.7% and show porhpyroblastic textures with granulation and kinking structures. Kizildağ harzburgites obtain equigranular textures with a larger degree of serpentinization (L.O.I.~10%). From depletion trends observed from mineral chemistry, the Kizildağ harzburgites are as depleted as the most depleted abyssal peridotites, while the Guleman harzburgites are even more depleted, in a degree similar to those fore-arc peridotites from the Izu-Bonin-Mariana (IBM) arc system. Trace element modeling not only suggests an approximately 20% melting of a refractory source that has already undergone around 16% melting of DMM, but also show enrichments in LREEs and LILEs. This 2-stage melting scenario is consistent with the modern petrogenetic model for boninites, hence we interpret these harzburgites as near pure melting residues after 2 stages of partial melting, with the latter possibly related to the generation of boninites in a fore-arc system. No significant metasomatism is documented in mineral chemistry, and only cryptic metasomatism was observed from trace element and PGE geochemistry. One of the Kizildağ harzbugite (KZD1301) are much more enriched in terms of trace elements than other samples, and combined with lines of evidence from petrography, spinel chemistry and trace element geochemistry, KZD1301 is intepreted in this study as residue that underwent a similar degree of melting with other KZD harzburgites but experienced more intense melt-rock interaction. Their Re-Os isotope ratios range from 0.1168 to 0.1271 for 187Os/188Os and from 0.0581 to 0.4819 for 187Re/188Os, the latter indicating recent Re enrichment. GI harzburgites obtain moderately subchondritic 187Os/188Os (~0.126) with flat PGE patterns slightly more depleted than the primitive mantle, while the 187Os/188Os of GII are more unradiogenic (~0.1234) and are subtly more enriched than primitive mantle in terms of PGE patterns. KZD harzburgite on the other hand, the least unradiogenic 187Os/188Os ratio (0.1168) from one Kizildağ peridotite with low 187Re/188Os of 0.0816, and refractory PGE pattern (Pd/IrN < 1) yields tRD age of 1.59 Ga. Compiling with available Os data of Guleman peridotites (Wang, K.-L., unpublished data), these peridotites yield TRD model age clusters of ~300 Ma, 650-700, ~800 Ma and 1600 Ma. Correlated with local crust forming events, the TRD age is coherent with rifting of the Neotethys (~250Ma). The remaining ancient Os model ages yielded may indicate other magmatic events related to juvenile crust formation documented in remnant lithospheric fragments isolated by disruption of the ancient continental regions during rifting (O’Reilly et al., 2009), or may just be reflecting mantle heterogeneity in Os isotopes.

並列關鍵字

Peridotite geochemistry ； Suprasubduction zone (SSZ) ； SE Turkey ； Tethyan orogeny ； Os isotopes ； Platinum Group Elements (PGE)

參考文獻

Iizuka, Y., Bellwood, P., Hung, H.C., Dizon, E.Z., 2005. A Non-destructive Mineralogical Study of Nephritic Artifacts from Itbayat Island, Batanes, Northern Philippines. Journal of Austronesian Studies, 1(1): 80-105.

Alard, O., Luguet, A., Pearson, N.J., Griffin, W.L., Lorand, J.P., Gannoun, A., Burton, K.W., O'Reilly, S.Y., 2005. In situ Os isotopes in abyssal peridotites bridge the isotopic gap between MORBs and their source mantle. Nature, 436(7053): 1005-8.

Allègre, C.J., Luck, J.M., 1980. Osmium isotopes as petrogenetic and geological tracers. Earth and Planetary Science Letters, 48: 148-154.

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土耳其東南部蛇綠岩套中超基性岩之地球化學特性及其隱沒帶特徵

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