The petrogenesis of Virgin Island granitoids: Implications for continental growth on the early Earth

The tectonic processes responsible for forming the oldest continental crust 4 billion years ago marked a new stage in the physical and geochemical evolution of the Earth's interior and surface. Initial continental growth would also have impacted on the evolution of the primitive hydrosphere-atmosphere, including the possible release of reduced volatile species that are the building blocks for prebiotic molecules. However, we still do not understand how the oldest continental crust began to form. This proposal sets out a research programme to investigate the tectonic settings in which the oldest continental crust was formed and the composition of its source rocks. We propose a cross-disciplinary and integrated strategy in which geochemical data from samples collected in the Caribbean Virgin Islands will inform high-pressure-temperature experiments coupled with thermodynamic and geodynamic modelling. Our results will deliver new insights into the formation of the oldest continental crust and give us a better understanding of the processes involved in the physical,
chemical and biological evolution of the early Earth.

The oldest continental crust is dominantly composed of tonalites, trondjhemites and granodiorites (TTG) (plagioclase-rich granites) that are derived from the partial melting of hydrous metamorphosed basaltic rocks followed by extensive magmatic differentiation processes. However, the tectonic setting and the composition of the basaltic source are controversial topics. Proposed tectonic settings on the early Earth include relatively deep plate tectonic-like processes and a variety of relatively shallow intraplate processes such as crustal resurfacing; crustal overturn and sagduction. Petrological, geochemical and field studies show that the Earth's crust, prior to the growth of the continents, was basaltic and compositionally similar to Mesozoic oceanic plateaus and modern island arc basalts (IAB). Thus, early TTG could have been derived from oceanic plateau and IAB lithologies in subduction and a variety of non-subduction tectonic settings. The Virgin Islands are composed of both thick oceanic plateau and IAB crust that has been intruded by early-Earth-like TTG.

The Virgin Island crust, during TTG formation, was underlain by a hotter-than-ambient mantle with early Earth-like potential temperatures and the area was the site of both deep subduction and shallow intraplate tectonic processes. Early Archaean terrains commonly provide only a fragmentary record of the crust-forming processes on the early Earth. Hence, the fully accessible TTG plutons and crust in the Virgin Islands provides a unique natural laboratory for testing how the earliest TTG, and therefore the oldest continental crust, formed 4 billion years ago. Our aim is to combine modern thermochemical, geochemical and geodynamic techniques to identify the tectonic settings responsible for generating the early Earth-like Virgin Island TTG rocks. Our results will lead to a better understanding of how the earliest preserved continental crust was generated through the following objectives:

O1: Sample intrusions and their associated mafic xenoliths on all the Virgin islands.

O2: Classify the rocks, investigate magmatic differentiation processes (magma mixing, crustal assimilation, accumulation & fractional  crystallisation), determine the composition of the TTG parental magmas and identify the broad compositional nature of the source region(s).

O3: Investigate the detailed compositional variability of the source region(s) and pressure-temperature conditions responsible for generating the  parental magmas that subsequently ascend and crystallise as TTG.

O4: Place the melt generation region of the TTG magmas in a geodynamic framework and determine if the TTG magmas formed in subduction and  intraplate environments. Combine geochemical and geodynamic results into a fully integrated model for the formation of continental crust on the Earth 4 billion years ago.

Our Earth is 4.56 billion years old and is unique among known planets in having a life-supporting atmosphere, liquid-water oceans, and continents made of silica-rich granitic crust that cover about forty percent of its surface. The remaining sixty percent of the crustal surface is covered by oceanic crust, which is thinner, composed of silica-poor basaltic rock, and is continually renewed.

Plate-tectonic processes constantly re-shape the Earth's surface today by forming new oceanic crust at mid-ocean ridges and destroying it by subduction beneath adjacent plates on time scales of less than 200 million-years. Oceanic crust is hydrated by interaction with sea water and this water is then released as the crust is dragged down into the Earth's interior, to trigger subduction-related magmatism as, for examples, Mt. Fuji and Mt. St. Helens, part of the Pacific Ring of Fire. The resulting silica-rich magmas solidify as igneous rocks and are then recycled through erosion, deposition and mountain building processes to create new continental crust. However, although plate tectonics explains crust formation on the present-day Earth, the tectonic processes operating on the early Earth 4 billion years ago are very poorly understood. The lack of knowledge about early crust-forming processes, and the influences that these processes may have had on early environments, means that we do not know (1) when plate tectonics started on Earth, (2) how the Earth's surface differentiated, (3) how early surface environments were chemically modified and (4) why life, especially the organisms that colonised the early land surface, was able to evolve.

The key for understanding how the early Earth developed and how it evolved into the modern world lies in the formation of the continental landmasses. This is because, over the last 4 billion years the growth, preservation, and erosion of the Earth's continental crust has been responsible for chemically modifying the planet's interior, crustal surface, and ocean atmosphere environments. However, there is still no consensus on how the oldest continental crust formed. Before the existence of the continents, magma from the Earth's interior solidified to form a thick world-wide basaltic crust. Around 4.0 billion years ago, at about the same time as the earliest life appeared in the oceans, this basaltic crust somehow started to re-melt to form the oldest preserved continental rocks. These continental rocks are predominantly composed of distinctive granite-like rocks called tonalites, trondhjemites and granodiorites (TTG) that are rarely formed on the Earth today. The tectonic settings and the composition of the source regions responsible for generating early Earth TTG remain controversial and unknown. What is required is to study an area of TTG formation that is associated with multiple source regions and tectonic environments to determine how the TTG could have formed. Here, we will investigate relatively modern early Earth-like TTG on the Virgin Islands of the north eastern Caribbean because the islands are composed of igneous rocks formed from multiple source regions at depth and are associated with several tectonic environments. Significantly, unlike the fragmentary record of crust-forming processes preserved in early Earth terrains, TTG rocks and their sources are fully accessible in the Virgin Islands. Therefore, the Virgin Islands provide a unique natural laboratory for testing how the earliest TTG, and therefore the oldest continental crust, formed 4 billion years ago.