地質學家提出地球早期的陸地是由地函柱形成，而非板塊碰撞

 原文網址：https://phys.org/news/2025-07-geologists-early-continents-mantle-plumes.html

近日香港大學的地質學家對於25億多年前稱為太古宙的遠古時期當中，地球的早期陸地是如何形成的這道問題有了新的進展。他們發表在《科學前緣》(Science Advances)的發現提出早期的大陸地殼可能是由發生在地球深處，稱為地函柱的作用所形成，而非今日塑造陸地的板塊構造作用。

不同於太陽系的其他行星，地球是唯一具有大陸地殼的。這些成分為花崗岩類的廣大陸塊使得生命可以生存，然而，大陸地殼的起源仍然是道懸而未解的謎題。科學家長久以來都在爭論早期的大陸地殼是否由板塊構造作用所形成，也就是分裂成巨大岩體的地殼彼此之間的隱沒與碰撞作用，或者是經由其他跟板塊運動無關的作用所形成。

此研究的主持人為趙丁一與王祥松博士，他們是香港大學地球與行星科學系趙國春教授早期地球研究團隊的成員。在與其他國際學者的合作之下，他們發現的強烈證據顯示有一種特殊的地球動力學機制形塑了地球的幼年期。

研究指出該機制並不是我們今日所見的板塊構造作用，而是一種地函柱——從地球深處上升、由高溫熔岩形成的巨型柱狀結構——所主導的體系。此外，研究也找到了重力沉降(sagduction)作用發生的痕跡，此現象是指地球表面的岩石因為自身重量逐漸沉到溫度較高的地球內部的過程。這些發現為地球早期岩石圈的演化是由哪些動力作用所主導帶來了新的見解。

研究古老的岩石來瞭解遠古時代

團隊分析的遠古花崗岩類稱為TTG(奧長花崗岩-英雲閃長岩-花崗閃長岩)，最古老的大陸地殼大部分都是由此種岩石組成。在中國北部具有年代為25億年左右的TTG，研究人員運用最新的技術來探討其中的微小鋯石顆粒，這種礦物可以保存岩石形成當時的化學訊號。

團隊測量了鋯石的水含量與氧同位素組成之後，發現這些岩石形成於乾燥的高溫環境。但是板塊互相碰撞使得其中一方沉到另一方之下的區域(隱沒帶)當中，通常不會看到這種環境。此外，氧同位素也顯示了融化的海洋岩石和沉積物互相混和的訊號，這也符合形成於地函柱上方的岩石所具有的特徵，而不是形成於隱沒帶的岩石。

研究人員提出一個兩階段的模型來解釋他們的發現。首先在大約27億年前，有根地函柱造成海床上方形成了非常厚的玄武岩(富含鐵和鎂的火山岩)。接著在大約25億年前，另一個地函柱帶來的高熱使得這塊玄武岩底部產生了部分熔融。此過程產生了密度較低的TTG岩石，最終形成了大陸地殼。

對於地球與行星科學的意義

「我們的結果強力證明了太古宙的大陸地殼並非一定要經由隱沒作用才能形成，」趙丁一博士解釋。這位香港大學地球與行星科學系的博士後研究員是此論文的第一作者。「反之，由地函柱上湧與重力沉降參與的兩階段過程，更能解釋在東部地塊觀察到的地球化學與地質特徵。」

這項研究比較了鋯石的水含量與氧同位素之後，將兩個同時代的太古宙TTG岩套區分開來：一個跟地函柱有關，另一個則跟火山弧有關。趙國春教授強調：「相較於華北中部造山帶，在隱沒帶上部區域所形成的TTG，東部地塊的TTG含有的水分明顯較少，這也更加鞏固了它並非產生自隱沒作用的解釋。」

「這項成果對於早期地球動力學來說具有重大貢獻，」共同作者，華盛頓大學的教授滕方振加以說明。「我們利用鋯石水含量與氧同位素，提出了一種相當實用的全新角度來研究早期大陸地殼的形成與演化。」

此研究不只讓我們對於太古宙大陸地殼的形成過程有了全新的理解，也凸顯出基於水含量所得出的代用指標在區分構造環境時的應用價值。連同這項成果在內，越來越多證據指出地函柱在早期大陸地殼的形成過程中扮演了相當重要的腳色。

 

Geologists suggest early continents formed through mantle plumes, not plate collisions

Geologists from the University of Hong Kong (HKU) have made a breakthrough in understanding how Earth's early continents formed during the Archean time, more than 2.5 billion years ago. Their findings, recently published in Science Advances, suggest that early continental crust likely formed through deep Earth processes called mantle plumes, rather than the plate tectonics that shape continents today.

Unlike other planets in our solar system, Earth is a unique planet with continental crust—vast landmasses with granitoid compositions that support life. However, the origin of these continents has remained a mystery. Scientists have long debated whether early continental crust formed through plate tectonics, i.e., the subduction and collision of giant slabs of Earth's crust, or through other processes that do not involve plate movement.

This study, led by Drs Dingyi Zhao and Xiangsong Wang in Mok Sau-King Professor Guochun Zhao's Early Earth Research Group at the HKU Department of Earth and Planetary Sciences, together with international collaborators, has uncovered strong evidence that a distinct geodynamic mechanism shaped Earth's formative years.

Rather than the plate tectonic processes we see today, the research points to a regime dominated by mantle plumes—towering columns of hot, molten rock ascending from deep within Earth. It also identifies a phenomenon known as sagduction, wherein surface rocks gradually descend under their weight into the planet's hotter, deeper layers. These findings shed new light on the dynamic processes that governed the early evolution of Earth's lithosphere.

Studying ancient rocks to understand the deep past

The team analyzed ancient granitoid rocks called TTGs (tonalite–trondhjemite–granodiorite), which make up a large part of the oldest continental crust. These rocks, found in northern China, date back around 2.5 billion years. Using advanced techniques, the researchers studied tiny minerals within the rocks, known as zircons, which preserve chemical signatures from the time the rocks were formed.

By measuring the water content and oxygen isotope composition of these zircons, the team found that the rocks were formed in dry, high-temperature environments, unlike those typically found in zones where tectonic plates collide and one sinks below the other (subduction zones). The oxygen signatures also indicate a mixture of molten oceanic rocks and sediments, consistent with rocks formed above mantle plumes rather than subduction zones.

The researchers proposed a two-stage model to explain their findings. About 2.7 billion years ago, a mantle plume caused thick piles of basalt (Fe- and Mg-rich volcanic rock) to form on the seafloor. Then, about 2.5 billion years ago, another mantle plume brought heat that caused the lower parts of these basalts to melt partially. This process produced the lighter TTG rocks that eventually formed continental crust.

Implications for Earth and planetary science

"Our results provide strong evidence that Archean continental crust did not have to be formed through subduction," explained Dr. Dingyi Zhao, postdoctoral fellow of the Department of Earth and Planetary Sciences and the first author of the paper. "Instead, a two-stage process involving mantle plume upwelling and gravitational sagduction of greenstones better explains the geochemical and geological features observed in the Eastern Block."

The study distinguishes between two coeval Archean TTG suites—one plume-related and the other arc-related— by comparing their zircon water contents and oxygen isotopes. Professor Guochun Zhao emphasized, "The TTGs from the Eastern Block contain markedly less water than those formed in a supra-subduction zone in the Trans-North China Orogen, reinforcing the interpretation of a non-subduction origin."

"This work is a great contribution to the study of early Earth geodynamics," co-author Professor Fang-Zhen Teng from the University of Washington added. "Our uses of zircon water and oxygen isotopes have provided a powerful new window into the formation and evolution of early continental crust."

This study not only provides new insights into understanding the formation of Archean continental crust, but also highlights the application of water-based proxies in distinguishing between tectonic environments. It contributes to a growing body of evidence that mantle plumes played a major role in the formation of the early continental crust.

原始論文：Dingyi Zhao, Peter A. Cawood, Fang-Zhen Teng, Guochun Zhao, Xiao-Ping Xia, Min Sun, and Xiangsong Wang. A two-stage mantle plume-sagduction origin of Archean continental crust revealed by water and oxygen isotopes of TTGs. Science Advances, 2025; 11 (24) DOI: 10.1126/sciadv.adr951

引用自：Phys.org. “Geologists suggest early continents formed through mantle plumes, not plate collisions”