Zircon inheritance in an igneous rock suite from the southern Adamello batholith (Italian Alps)
Zircon inheritance in an igneous rock suite from the southern Adamello batholith (Italian Alps)
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DOI：
10.1007/bf00310684
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发表时间：
1991-06
影响因子：
3.5
通讯作者：
W. Hansmann;F. Oberli
中科院分区：
文献类型：
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作者：
W. Hansmann;F. Oberli
来自意大利南阿尔卑斯Adamello岩基（S AB）南部的一套碱性-酸性钙碱性火成岩锆石显示出复杂的U - Pb同位素模式，这主要是由于不同数量的同位素非均质遗传放射性成因Pb的存在，以及少量的岩浆后Pb损失。继承Pb主要由1)位于无可见岩心锆石中的~ 1100 Ma Pb组分和2)与明显的富气泡浑浊岩心相关的~ 450 Ma Pb组分组成。在concordia表示中，数据点符合~ 40 Ma的较低截距年龄。对三个没有可见花岗闪长岩岩心的样品进行线性拟合，确定了入侵年龄为~ 39.3 Ma。锆石的U - Pb分系统（特别是U含量）与晶体形态有明显的相关性。G1型锆石在锆石结晶序列中形成较晚，在各锆石群中U含量一致最高。然而，这些晚期锆石并非没有遗传的放射性成因铅。在花岗闪长岩种群中，岩心在整个形态谱中是随机分布的。所有研究样品中存在古老的继承锆石成分，为地壳物质参与sab岩石的成因提供了证据。以地壳ε nd和ε sr值为特征的样品，锆石的继承性增强。遗传随分化而变化，中酸性成员遗传最大；这些成分显示了所研究的岩石谱计算出的最高Zr饱和温度。锆石与主要相的结构关系表明，白岩浆岩的岩浆在结晶早期就已饱和Zr。另一方面，Zr溶解度和结构关系一致表明，基性-中间岩熔体不饱和Zr。将Zr溶解度模型推广到矿物/熔体混合物中，发现斜长石和角闪洞的结晶作用大大降低了残余熔体中Zr的溶解度。异晶锆石在S AB基性至中间岩中的存活，最好用溶解动力学来解释。由于这些锆欠饱和熔体的温度和水含量有利于相对快速的锆石溶解，因此继承的锆石（特别是富含微量元素的不稳定岩心）不可能在这种条件下长时间暴露。因此，南大陆架的调性或基性岩浆不可能是通过埋藏变质作用等缓慢过程从地壳源中产生的。
Zircons from a suite of basic to acidic calcalkaline igneous rocks from the southern Adamello batholith (S AB), Southern Alps, N Italy, display complex U−Pb isotopic patterns which are mainly due to the presence of variable amounts of isotopically heterogeneous, inherited radiogenic Pb, and to minor postmagmatic loss of Pb. Inherited Pb is mainly composed of 1) a ∼ 1100 Ma Pb component located in zircons devoid of visible cores and 2) a ∼ 450 Ma component associated with conspicuous bubble-rich turbid cores. In concordia representation the data points conform to lower intercept ages of ∼ 40 Ma. A linear fit of three samples devoid of visible cores from the granodiorite defines an intrusion age of ∼ 39.3 Ma. U−Pb systematics of zircon (in particular U content) and crystal morphology are clearly related. Zircons of type G1, which form relatively late in the zircon crystallization sequence, consistently show the highest U contents in each zircon population. These late zircons, however, are not devoid of inherited radiogenic lead. In a population from a granodiorite, cores are randomly distributed throughout the morphological spectrum. The presence of old inherited zircon components inallinvestigated samples furnishes proof for involvement of crustal material in the genesis of the S AB rocks. Samples characterized by crustal ɛNdand ɛSrvalues usually show enhanced zircon inheritance. Inheritance varies with differentiation and reaches a maximum for intermediate to acidic members; these compositions show the highest Zr saturation temperatures calculated for the rock spectrum studied. Textural relations between zircon and major phases indicate that the magmas of the leucocratic rocks were saturated with Zr at an early stage of crystallization. On the other hand, Zr solubilities and textural relations consistently show, that melts of basic to intermediate rocks were not saturated with Zr. Extension of the Zr solubility model to mineral/melt mixtures of tonalitic bulk composition demonstrates that Zr solubility in the residual melt is drastically reduced by crystallization of plagioclase and amphibole. Survival of xenocrystic zircons in the mafic to intermediate rocks of the S AB can best be explained in terms of dissolution kinetics. Since temperature and H2O content of these Zr-undersaturated melts were favourable for relatively rapid zircon dissolution, inherited zircons (in particular trace-element rich unstable cores) cannot have been exposed to such conditions over extended time periods. Therefore, the tonalitic or more basic magmas of the S AB cannot have been derived from crustal sources by slow processes such as burial metamorphism.