山东郯城神泉钠质火山岩的年代学与地球化学——对源区地幔性状与岩石成因的启示

Geochronology and geochemistry of sodic volcanic rocks from Shenquan in Tancheng County,Shandong Province: Implications for unraveling the nature of mantle source and petrogenesis

山东神泉钠质火山岩出露于郯城李庄东侧,构造上位于沂沭断裂带南段,构成一盾状火山机构,主体岩性为粗安岩。锆石LA-ICP-MS U-Pb定年结果显示,其成岩年龄为96.5±1.4 Ma,较之区内广泛分布的钾质火山岩形成年龄晚,指示沂沭断裂带及邻区中生代火山岩总体上具有由钾质向钠质演变的趋势。在化学组成上,神泉火山岩具有富碱(Na2O+K2O=10.34%~11.95%)和明显富钠(Na2O/K2O=1.76~3.92)的特征,按多种方法判别均可归为典型的钠质火山岩。该火山岩稀土元素总量较高(ΣREE=411.3×10-6~456.3×10-6),富集轻稀土元素,LREE/HREE=19.53~21.10,(La/Yb)N=29.69~33.26,缺乏明显的铕异常(δEu=0.77~0.82),稀土元素配分型式呈明显的右倾型。岩石富Rb、Ba、Th等大离子亲石元素,亏损Ti、Nb、Ta等高场强元素,在微量元素蛛网图上显示一定的Pb负异常。神泉火山岩具有富集的Sr-Nd同位素特征,ISr=0.706 8~0.707 7,εNd(t)=-16.66^-16.82,与山东中生代镁铁质岩石的Sr-Nd同位素组成类似,表明其应起源于富集的岩石圈地幔。综合分析表明,神泉火山岩源区极可能遭受了因扬子板块深俯冲和华北克拉通岩石圈拆沉等多次富集事件的影响,至晚白垩世,随着郯庐断裂引张的加剧,诱发了软流圈地幔上涌,并对先期形成的富集地幔进一步改造,同时在断裂减压作用的影响下,促使了这一经叠次改造的富集地幔的部分熔融,由此产生的岩浆经进一步分异演化,最终导致了神泉钠质火山岩的形成。

The Shenquan sodic volcanic rocks, outcropped in the east of Lizhuang village in Tancheng County, Shandong Province, are tectonica[ly located in the southern parts of the Yishu deep fault zone. The volcanic rocks comprise a shield volcano, and lithologically consist mainly of trachyandensite. Zircon LA-ICP-MS U-Pb dating yields an age of 96.5 ＋ 1.4 Ma, which is younger than the ages of the widespread potassic rocks within the region, indicating an evolutionary trend from potassic to sodic for the Mesozoic volcanic rocks along the Yishu deep fault zone and its adjacent areas. Chemically, the Shenquan volcanic rocks have high alkali content （Na20 ＋ K20= 10.34% -- 11.95% ） and are obviously enriched in sodium （Na20/K20 = 1.76-- 3.92） ; they can be assigned to typical sodic volcanic rocks on the basis of various discrimination schemes. Geochemically, the rocks contain relatively high concentrations of rare earth elements （REE = 411.3 × 10^6-456.3× 10^-6）, and are enriched in LREE [LREE/HREE= 19.53--21.10, （La/Yb）N=29.69--33.26] and LILE （e. g., Rb, Ba, Th）, and depleted in HFSE （e. g., Ti, Nb, Ta）. They show right-oblique chondrite-normalized REE patterns and display some negative Pb anomalies in the primitive mantle-normalized trace element spidergrams. These rocks have enriched Sr-Nd isotopic compositions [ Isr = 0. 706 8 -- 0. 707 7, eNd （ t ） = - 16.66 - 16.82], similar to that of the Mesozoic mafic rocks in Shandong, indicating that they were probably derived from an enriched lithospheric mantle. Based on a synthesis of geology and geochemistry, the authors hold that the mantle source of the Shenquan sodic volcanic rocks might have undergone multiple enrichment events induced by source contamination of continental crustal materials both from deep subduction of the Yangtze Craton and from lithospheric delamination of the North China Craton. During Late Cretaceous, the continued extension of Tanlu Fault induced the upwelling of asthenospheric melts which further modified the previously enriched mantle. Decompression partial melting of these multiply-modified mantle sources and further differentiation of the magmas finally generated the Shenquan sodic volcanic rocks.