Abstract

Coupling of the geological processes in the arc-magmatic and back-arc provinces of accretionary orogens in Neoarchean plate tectonic setting is a subject of current research all over the world. The Dharwar Craton of southern India is an example of such an accretionary orogen, with an arc-magmatic province in the east and a back-arc province in the west, referred as the Eastern Dharwar Craton and the Western Dharwar Craton, respectively. The boundary between the two provinces is considered to be marked by a 400-km long shear zone along the eastern margin of the Gadag–Chitradurga–Karighatta greenstone belt which is called as the Chitradurga Boundary Shear Zone. Potassic, metaluminous, I type, calc-alkalic to alkali-calcic, arc-magmatic granitoids are widespread in the EDC. But they are also found to occur along the western margin of the Chitradurga–Gadag greenstone belt. SHRIMP U–Pb zircon ages of the granitoids in the western back-arc province in the Gadag region occurring near Srimant Gudda, Mulgund and Chabbi have been determined as 2565 ± 24 to 2591 ± 64 Ma old. Within errors, the ages of these granitoids are the same as the Lakundi and Turchihal granitoids occurring to the east of the Gadag Greenstone Belt in the arc-magmatic province. Nd isotope systematics of the granitoids suggest that they were formed from magmatic melts that were produced by remelting of 3200–3500 Ma old heterogenous continental crust. Rare inherited zircons support this antiquity of the protoliths. Occurrence of granitoids of similar age and origin, in the western back-arc province and eastern arc-magmatic province in the Gadag area was attributed to thrust duplex structure in the Gadag region. However, elsewhere, along the western margin of the Chitradurga greenstone belt near Harpanhalli, Hosdurga–Nagamangala–Pandavapura sector, or away from it in the Arsikere–Banavara, where repetition by thrusting is not obvious, late potash arc-magmatic type granitoids of similar age as the Gadag arc-magmatic granitoids are observed. The arc-magmatic type granitoids appear to have overstepped the boundary shear into the back-arc province at several places. Arc-magmatic and back-arc boundary may be diffuse rather than sharp, as also suggested by some earlier workers.

Research highlights

• The ages and petrogenesis of the granitoids around GGB of both the sides of EDC (i.e., eastern magmatic-arc province) and WDC (i.e., western back-arc provinces) are very much similar.

• The geochemistry and isotope systematics (i.e., Nd TDM2 ages and εNdT values at 2.5–2.6 Ga) of the granitoids exposed one both the sides are also similar.

• This shows not only a similar antiquity of both the provinces, but also gives evidence for a possible diffusive nature of boundary between the EDC and the WDC around GGB.

中文翻译：

---

新太古代花岗岩的锆石 SHRIMP U-Pb 年代学、地球化学和 Nd 同位素系统学，Gadag 绿岩带，Dharwar Craton，印度南部：岩石成因和构造意义

摘要

新太古代板块构造背景下增生造山带弧-岩浆区和弧后区地质过程的耦合是当今世界范围内的研究课题。印度南部的达尔瓦尔克拉通就是这种增生造山带的一个例子，东部有一个弧岩浆省，西部有一个弧后省，分别称为东达尔瓦尔克拉通和西达尔瓦尔克拉通。两省之间的边界被认为以沿 Gadag-Chitradurga-Karighatta 绿岩带东缘的 400 公里长剪切带为标志，称为 Chitradurga 边界剪切带。钾质、金属铝质、I 型、钙碱性到碱钙质、弧岩浆花岗岩在 EDC 中广泛分布。但也发现它们出现在 Chitradurga-Gadag 绿岩带的西缘。Srimant Gudda、Mulgund和Chabbi附近Gadag地区弧后西部省花岗岩的SHRIMP U-Pb锆石年龄已确定为2565±24至2591±64Ma。在误差范围内，这些花岗岩的年龄与发生在弧岩浆省加达格绿岩带以东的 Lakundi 和 Turchihal 花岗岩相同。花岗岩的Nd同位素系统表明，它们是由3200-3500 Ma老异质大陆壳重熔产生的岩浆熔体形成的。罕见的继承锆石支持这种古老的原岩。出现类似年龄和起源的花岗岩，Gadag 地区弧后西部省和弧东部岩浆省的逆冲断层归因于 Gadag 地区逆冲双相构造。然而，在其他地方，沿着靠近 Harpanhalli 的 Chitradurga 绿岩带的西缘，Hosdurga-Nagamangala-Pandavapura 扇区，或远离它的 Arsikere-Banavara，在那里逆冲重复不明显，类似的晚期钾盐弧岩浆型花岗岩Gadag 弧岩浆花岗岩的年龄。弧岩浆型花岗岩似乎在几个地方超过了边界剪切进入弧后区。正如一些早期工作人员所建议的那样，弧岩浆和弧后边界可能是弥散的而不是尖锐的。沿 Chitradurga 绿岩带西缘靠近 Harpanhalli、Hosdurga-Nagamangala-Pandavapura 扇区，或远离它的 Arsikere-Banavara，逆冲重复不明显，与 Gadag 年代相近的晚期钾盐弧岩浆型花岗岩观察到弧岩浆花岗岩。弧岩浆型花岗岩似乎在几个地方超过了边界剪切进入弧后区。正如一些早期工作人员所建议的那样，弧岩浆和弧后边界可能是弥散的而不是尖锐的。沿 Chitradurga 绿岩带西缘靠近 Harpanhalli、Hosdurga-Nagamangala-Pandavapura 扇区，或远离它的 Arsikere-Banavara，逆冲重复不明显，与 Gadag 年代相近的晚期钾盐弧岩浆型花岗岩观察到弧岩浆花岗岩。弧岩浆型花岗岩似乎在几个地方超过了边界剪切进入弧后区。正如一些早期工作人员所建议的那样，弧岩浆和弧后边界可能是弥散的而不是尖锐的。弧岩浆型花岗岩似乎在几个地方超过了边界剪切进入弧后区。正如一些早期工作人员所建议的那样，弧岩浆和弧后边界可能是弥散的而不是尖锐的。弧岩浆型花岗岩似乎在几个地方超过了边界剪切进入弧后区。正如一些早期工作人员所建议的那样，弧岩浆和弧后边界可能是弥散的而不是尖锐的。

研究亮点

• EDC（即东部岩浆弧区）和WDC（即西部弧后区）两侧GGB周围花岗岩的年龄和成因非常相似。

• 两侧出露的花岗岩的地球化学和同位素系统学（即Nd TDM2年龄和εNdT值在2.5-2.6 Ga）也相似。

• 这不仅表明这两个省的历史相似，而且还证明了 EDC 和 WDC 之间围绕 GGB 的边界可能具有扩散性质。