山东汤头盆地钾质及钠质火山岩的年代学与地球化学:对华北克拉通岩石圈减薄的启示

山东汤头盆地位于沂沭断裂带南段,盆地内广泛发育以粗安质岩石为主体的晚中生代火山岩,这套岩石主要可归为碱性系列,按化学组成可进一步区分为钾质和钠质二种类型。钾质火山岩的主要岩性为黑云母粗安质火山碎屑岩和潜火山岩,钠质火山岩主要为辉石粗安质潜火山岩,其中钾质火山岩是盆地内火山岩的主体。锆石LA-ICP-MS U-Pb定年获得钾质和钠质火山岩的成岩年龄分别为124.0±1.3Ma和106.4±4.0Ma,表明钠质火山岩较钾质火山岩形成晚。在化学组成上,钾质火山岩较钠质火山岩全碱含量更高,二者的K2O+Na2O含量分别为11.02%～11.37%和8.75%～8.93%。它们均富轻稀土和大离子亲石元素,但钾质火山岩较钠质火山岩稀土总量更高,且更富轻稀土,二者的∑REE值分别为360.1×10-6～417.0×10-6和232.3×10-6～291.0×10-6,(La/Yb)N比值分别为62.02～64.66和40.32～40.52。钾质火山岩的Cs、Rb、Ba、Th、U、Pb等大离子亲石元素和Zr、Hf等高场强元素均较钠质火山岩富集,但Sr、Ti的含量偏低。钾质与钠质火山岩均具有富集的Sr-Nd同位素组成特征,但钠质火山岩的ISr值偏低、而εNd(t)值偏高,二者的ISr和εNd(t)值分别为0.7107～0.7119和-15.48～-16.96,以及0.7098和-10.03。元素和Sr-Nd同位素组成的系统分析表明浅部地壳混染对火山岩地球化学特征未产生显著影响,二类火山岩地球化学特征的变异应主要受控于岩浆源区组成的不同,而不是岩浆演化过程的差异所致。二元混合模拟指示二类火山岩均最可能起源因华北克拉通下地壳拆沉而形成的富集地幔的熔融,但钠质火山岩源区含有较高比例的亏损软流圈地幔组分。根据对火山岩地质与地球化学特征的综合分析,表明郯庐断裂持续的引张促使岩石圈减薄,并诱发深部软流圈熔体上涌,这一上涌的软流圈熔体随后又与原先富集的岩石圈地幔混合,从而导致晚期的钠质火山岩源区中含有较高的亏损软流圈地幔组分。火山岩成分由钾质向钠质演化,是软流圈地幔上涌并置换原有岩石圈地幔,最终导致华北克拉通减薄的直接响应。

Geochronology and geochemistry of potassic and sodic volcanic rocks in Tangtou basin, Shandong Province: Implications for lithospheric thinning beneath the North China Craton

The Tangtou basin in Shandong Province is located in the southern segment of the Yishu fault zone,Late Mesozoic trachyandesitic volcanic rocks belonging to alkaline series are widely developed in this basin,Chemically,these volcanic rocks can be further distinguished into a potassic type and a sodic type,of which the former is the predominant rocks in the basin,The potassic type consists mainly of biotite trachyandesitic pyroclastic rocks and subvolcanic analogues,and the sodic type comprises mainly of pyroxene trachyandesitic subvolcanic rocks,Zircon LA-ICP-MS U-Pb dating yields an age of 124.0±1.3Ma for the potassic rocks,and an age of 106.4±4.0Ma for the sodic rocks,indicating that the sodic rocks were crystallized in a late stage,The potassic and the sodic volcanic rocks show systematically geochemical differences,Compared with the sodic volcanic rocks,the potassic rocks are more enriched in alkalis,with K2O+Na2O contents of 11.02%～11.37% and of 8.75%～8.93%,respectively,All the volcanic rocks are enriched in LREE and LILE,but the potassic rocks have higher REE(especially LREE) contents relative to the sodic rocks,with ∑REE values of 360.1×10-6～417.0×10-6 and(La/Yb)N ratios of 62.02～64.66 for the potassic rocks and of 232.3×10-6～291.0×10-6 and 40.32～40.52 for the sodic ones,The potassic rocks also show higher LILE(e.g.,Cs,Rb,Ba,Th,U,Pb,etc.) and HFSE(e.g.,Zr,Hf) concentrations,but lower Sr,Ti abundances,All the samples have enriched Sr and Nd isotopic compositions,but the sodic rocks have slightly lower ISr ratios and relatively higher εNd(t) values(specially 0.7107～0.7119 and-15.48～-16.96 for the potassic rocks,and 0.7098 and-10.03 for the sodic rocks),The elemental and Sr-Nd isotope systematics show that shallow-level crustal contamination did not contribute significantly to their geochemical characteristics,suggesting that the source differences rather than magmatic evolution processes could be a major controlling factor for their geochemical variations,Binary mixing simulation demonstrates that the two type volcanic rocks were generated by melting of enriched lithospheric mantle which was most likely induced by lithospheric delamination of the lower crust of the North China Craton,but the magma source of the sodic rocks contain a higher proportion of depleted asthenosphere mantle components,Based on a synthesis of the geology and geochemistry,we suggest that the continued extension of the Tanlu fault thinned the lithosphere and induced the upwelling of asthenospheric melts which then mixed with previously enriched lithospheric mantle,thus resulted the source region of the late sodic volcanic rocks contained a higher proportion of depleted asthenosphere mantle components,The chemical compositions of volcanic rocks evolved from potassic to sodic directly responses to the replacement of lithosphere mantle by the upwelling asthenosphere mantle,which ultimately led the lithospheric thinning of the North China Craton.