Evolution of an intra‐oceanic island arc during the Late Silurian to Late Devonian, New England Fold Belt
Evolution of an intra‐oceanic island arc during the Late Silurian to Late Devonian, New England Fold Belt
复制标题
DOI：
10.1046/j.1440-0952.2002.00923.x
复制
发表时间：
2002-04
影响因子：
1.2
通讯作者：
R. Offler;J. Gamble
中科院分区：
文献类型：
--
作者：
R. Offler;J. Gamble
澳大利亚东部新英格兰褶皱带Gamilaroi地体和Calliope火山岩组合中火山岩的地球化学研究表明，这些岩石形成的环境在空间和时间上都发生了变化。Gamilaroi地体上志留统至中泥盆统玄武岩具有扁平-轻度轻稀土元素(LREE)亏损的球粒陨石标准化模式，高场强元素(HFSE)相对N-MORB亏损，低Ti/V和高Ti/Zr比值，高Ni、Cr和大离子亲石元素(LILE)含量，具有大洋岛弧玄武岩岩浆的特征。它们与低K、低镁铁质火山岩有关，具有中等的LREE富集度、低的Nb、Y含量和Rb/Zr比值。玄武岩中高场强元素的亏损表明岩浆来自于超俯冲带环境中的耐火源。这样一个带的存在意味着该弧与一个弧后盆地有关，该盆地的位置在西边，从中志留世起存在一个宽阔的弧后区域。弧和弧后盆地的这种极性表明俯冲带向西倾斜。与更老的玄武岩相比，Gamilaroi地体中-上泥盆统玄武岩具有类似MORB的球粒陨石标准化模式，并且具有较高的钛和较低的LILE含量。此外，它们具有弧后盆地的典型特征--低的钛/锆比值、类MORB的钛/钒比值和高场强元素含量。加米拉罗伊地体的榴辉岩也具有主要的弧后盆地特征。这些特征表明玄武岩和榴辉岩都是在大洋内岛弧岩石圈裂谷过程中形成的伸展环境中侵位的。在地层上相当的玄武岩中，钛/钒比以及在恒定的镁含量下的二氧化钛和三氧化二铁的含量逐渐增加，向北延伸，或与更丰富的岩浆源区熔融一致。相比之下，地体东南部的玄武岩具有中等高的Ti/Zr和低的Ti/V比值，在某些样品中表现出高场强亏损，成分特征介于岛弧和弧后盆地玄武岩之间。Calliope火山组合中下泥盆统至中泥盆统镁铁质岩具有轻稀土富集型和亏损球粒陨石正常化稀土配分模式。此外，与MORB相比，大多数样品具有高的Ti/Zr比和低的Zr含量以及相对较高的Th含量。这些特征在中泥盆世和早泥盆世的岩石中都是常见的，并暗示着在弧形背景下喷发。因此，这项研究的数据为新英格兰褶皱带从晚志留世到晚泥盆世的演化提供了新的证据，并揭示了一段比以前报道的更复杂的历史。
Geochemical studies of volcanic rocks in the Gamilaroi terrane and Calliope Volcanic Assemblage, New England Fold Belt, eastern Australia, indicate that the setting in which these rocks formed changed in both space and time. The Upper Silurian to Middle Devonian basalts of the Gamilaroi terrane show flat to slightly light rare‐earth element (LREE) depleted chondrite normalised patterns, depletion of high field strength elements (HFSE) relative to N‐MORB, low Ti/V and high Ti/Zr ratios, high Ni, Cr and large‐ion lithophile element (LILE) contents, features characteristic of intra‐oceanic island arc basaltic magmas. They are associated with low‐K, less mafic volcanics, showing moderate LREE enrichment, low Nb and Y contents and Rb/Zr ratios. The depletion of HFSE in the basalts indicates that the magmas were derived from a refractory source in a supra‐subduction zone setting. The presence of such a zone implies that the arc was associated with a backarc basin, the location of which was to the west where a wide backarc region existed from the Middle Silurian. This polarity of arc and backarc basin suggests that the subduction zone dipped to the west. In contrast to their older counterparts, Middle to Upper Devonian basalts of the Gamilaroi terrane have MORB‐like chondrite normalised patterns and higher Ti and lower LILE contents. Moreover, they have low Ti/Zr ratios and MORB‐like Ti/V ratios and HFSE contents, features typical of backarc basins. Dolerites of the Gamilaroi terrane also have predominantly backarc basin signatures. These features suggest that both the basalts and dolerites have been emplaced in an extensional environment produced during the rifting of the intra‐oceanic island arc lithosphere. A progressive increase in Ti/V ratios, and TiO2 and Fe2O3 contents at constant MgO, of stratigraphically equivalent basalts, towards the north‐northwest part of the belt, is consistent with either greater extension to the north or melting of a more fertile magma source. By contrast, basalts in the southeast part of the terrane have moderately high Ti/Zr and low Ti/V ratios and in some samples, exhibit depletion of HFSE, compositional features transitional between island arc and backarc basin basalts. The Lower to Middle Devonian mafic rocks in the Calliope Volcanic Assemblage show both LREE enriched and depleted chondrite normalised REE patterns. Further, the majority have high Ti/Zr ratios and low Zr contents as well as relatively high Th contents relative to MORB. These features are common to rocks of Middle Devonian age as well as those of Early Devonian age and are suggestive of eruption in an arc setting. Thus, the data from this study provide new evidence for the evolution of the New England Fold Belt from the Late Silurian to the Late Devonian and reveal a history more complicated than previously reported.