地质论评
地質論評
지질론평
GEOLOGICAL REVIEW
2015年
3期
547-560
,共14页
刘成军%裴先治%李佐臣%李瑞保%裴磊%陈国超%陈有炘%徐通%张永明
劉成軍%裴先治%李佐臣%李瑞保%裴磊%陳國超%陳有炘%徐通%張永明
류성군%배선치%리좌신%리서보%배뢰%진국초%진유흔%서통%장영명
镁铁-超镁铁质岩%SSZ 型蛇绿岩%E-MORB%晚奥陶世%秦祁交接部位
鎂鐵-超鎂鐵質巖%SSZ 型蛇綠巖%E-MORB%晚奧陶世%秦祁交接部位
미철-초미철질암%SSZ 형사록암%E-MORB%만오도세%진기교접부위
mafic-ultramafic rocks%SSZ-type ophiolite%E-type MORB%Late Ordovician%Conjunction of the Qinling-Qilian orogenic belts
秦祁交接部位天水新阳镇细尾子沟新发现一套镁铁—超镁铁质岩，超镁铁质岩主要由蛇纹石化橄辉岩和透辉石岩组成，岩石具有低 SiO2含量、高 MgO 含量、稀土元素和微量元素与原始地幔含量相当的特点。镁铁质岩主要由变辉长岩和变辉长闪长岩组成，岩石具有堆晶岩的特点，具有低 SiO2和 TiO2含量、高 MgO 含量和 Mg＃值，具有较低的 Ti ／V 比值（20±）、较高的稀土元素丰度和 HREE 较平坦的稀土元素配分模式；镁铁质岩属于拉斑玄武岩系列，其岩浆源区是受地壳物质交代改造过的富集型岩石圈地幔，具有 E-MORB 的特征。综合构造环境和区域大地构造背景，认为细尾子沟镁铁—超镁铁质岩为较典型的岛弧 SSZ 型蛇绿岩。LA-ICP-MS 锆石 U-Pb 测年结果表明细尾子沟岛弧 SSZ 型蛇绿岩形成于晚奥陶世晚期（443．6±1．8Ma）。细尾子沟岛弧 SSZ 型蛇绿岩的形成与“天水—武山”古洋盆自南向北的俯冲作用密切相关，表明秦祁交接部位早古生代构造格局是一个完整的洋—弧—盆体系。
秦祁交接部位天水新暘鎮細尾子溝新髮現一套鎂鐵—超鎂鐵質巖，超鎂鐵質巖主要由蛇紋石化橄輝巖和透輝石巖組成，巖石具有低 SiO2含量、高 MgO 含量、稀土元素和微量元素與原始地幔含量相噹的特點。鎂鐵質巖主要由變輝長巖和變輝長閃長巖組成，巖石具有堆晶巖的特點，具有低 SiO2和 TiO2含量、高 MgO 含量和 Mg＃值，具有較低的 Ti ／V 比值（20±）、較高的稀土元素豐度和 HREE 較平坦的稀土元素配分模式；鎂鐵質巖屬于拉斑玄武巖繫列，其巖漿源區是受地殼物質交代改造過的富集型巖石圈地幔，具有 E-MORB 的特徵。綜閤構造環境和區域大地構造揹景，認為細尾子溝鎂鐵—超鎂鐵質巖為較典型的島弧 SSZ 型蛇綠巖。LA-ICP-MS 鋯石 U-Pb 測年結果錶明細尾子溝島弧 SSZ 型蛇綠巖形成于晚奧陶世晚期（443．6±1．8Ma）。細尾子溝島弧 SSZ 型蛇綠巖的形成與“天水—武山”古洋盆自南嚮北的俯遲作用密切相關，錶明秦祁交接部位早古生代構造格跼是一箇完整的洋—弧—盆體繫。
진기교접부위천수신양진세미자구신발현일투미철—초미철질암，초미철질암주요유사문석화감휘암화투휘석암조성，암석구유저 SiO2함량、고 MgO 함량、희토원소화미량원소여원시지만함량상당적특점。미철질암주요유변휘장암화변휘장섬장암조성，암석구유퇴정암적특점，구유저 SiO2화 TiO2함량、고 MgO 함량화 Mg＃치，구유교저적 Ti ／V 비치（20±）、교고적희토원소봉도화 HREE 교평탄적희토원소배분모식；미철질암속우랍반현무암계렬，기암장원구시수지각물질교대개조과적부집형암석권지만，구유 E-MORB 적특정。종합구조배경화구역대지구조배경，인위세미자구미철—초미철질암위교전형적도호 SSZ 형사록암。LA-ICP-MS 고석 U-Pb 측년결과표명세미자구도호 SSZ 형사록암형성우만오도세만기（443．6±1．8Ma）。세미자구도호 SSZ 형사록암적형성여“천수—무산”고양분자남향북적부충작용밀절상관，표명진기교접부위조고생대구조격국시일개완정적양—호—분체계。
The newly discovered Xiweizigou mafic—ultramafic rocks are cropped out in Xinyang town of Tianshui area at the conjunction of the Qinling—Qilian orogenic belts.The ultramafic rocks are composed of serpentinization olivine—pyroxenite and diopsidite,and characterized by low SiO2 and high MgO contents,and similar to that of primitive mantle in rare earth element and trace element.The mafic rocks are composed of meta-gabbro and meta-gabbrodiorite and have the characteristics of cumulates,and are characterized by low SiO2 and TiO2 contents,high MgO content and Mg#,low Ti /V ratio(20 ±),high abundances of rare earth elements and relatively flat in HREE.The mafic rocks belong to the tholeiitic series according to their geochemical characteristics.The abundance and variation of high field strength elements and ratios of associated elements suggest that the mafic rocks show characteristics of E-type MORB,and are derived from enriched lithosphere mantle which had been hybridized by the crust.Taking the tectonic setting and regional tectonic background into account,the geochemistry characteristics indicate that the Xiweizigou mafic—ultramafic rocks are relatively typical SSZ-type ophiolite related to arc—island setting.LA-ICP-MS zircon U-Pb dating yields a 206 Pb /238 U weighted average age of 443.6 ±1.8 Ma, indicating that the SSZ-type ophiolite should be formed in the Late Ordovician.The formation of the SSZ-type ophiolite is closely related to the northward subduction of Tianshui—Wushan Ocean,and suggest a tectonic framework with an integrated ocean—arc—basin system in the Early Paleozoic at the conjunction of the Qinling—Qilian orogenic belts.
