What is the Protolith of amphibolite?

Amphibolite’s Secret Past: What This Rock Used to Be Before It Got So… Metamorphic?

Ever picked up a dark, kinda heavy rock and wondered about its story? Well, amphibolite definitely has a tale to tell. It’s a metamorphic rock, meaning it’s been through some serious changes deep down in the Earth. But what exactly did it start out as? That’s where the idea of a “protolith” comes in – basically, the original rock before the metamorphic makeover. Knowing the protolith unlocks the secrets of amphibolite’s past and the wild geological ride it’s been on.

The Usual Suspect: Good Ol’ Mafic Igneous Rocks

Most of the time, amphibolite comes from mafic igneous rocks. Think of basalt and gabbro – those dark-colored rocks loaded with magnesium and iron, and not so much silica. You find them all over the place, especially in the ocean crust, but also kicking around on continents.

• Basalt: Imagine lava cooling super fast. That’s basalt – a fine-grained, extrusive rock.
• Gabbro: Now picture magma cooling slowly underground. That’s gabbro – a coarse-grained, intrusive rock.
• Dolerite/Diabase: A rock with a medium grain size.

So, what happens? During metamorphism – a process I like to think of as a geological pressure cooker – the minerals in basalt and gabbro (mainly pyroxene and plagioclase) get transformed. They morph into amphiboles like hornblende and actinolite, along with plagioclase feldspar. These are the key ingredients that define amphibolite. This usually happens at pretty extreme temperatures (500 to 750 °C) and pressures (5 to 10 kbar) – conditions we associate with the “amphibolite facies.” And you need water in the mix to help those amphibole minerals form. It’s like baking a cake; you need all the right ingredients and conditions!

Not Just Basalt: When Sediments Get in on the Act

Okay, mafic igneous rocks are the main source, but sometimes, just sometimes, amphibolite comes from sedimentary rocks. These are often called “para-amphibolites,” to set them apart from the “ortho-amphibolites” that come from igneous rocks.

• Dirty Marls: Think clay-rich, kinda messy carbonate sediments.
• Volcanic Sediments: Sedimentary rocks that contain volcanic material.
• Greywacke: A dark sandstone that is poorly sorted and contains rock fragments.
• Dolomite and Siderite Deposits: Under the right circumstances, even these carbonate-rich sediments can become amphibolite, especially when they’re close to a heat source.

Turning sedimentary rocks into amphibolite depends a lot on what the original rock was made of. It’s all about the right mix of impurities and minerals to allow those amphibole and plagioclase crystals to grow.

Cracking the Case: How Geologists Play Detective

Figuring out the exact protolith of an amphibolite? That can be tricky. Metamorphism can really mess things up, wiping out the original textures and structures. It’s like trying to recognize someone after they’ve had major plastic surgery! But geologists have some cool tools:

• Mineral Composition: What minerals are in the rock, and how much of each? Para-amphibolites tend to have more biotite, quartz, and maybe even calcite/aragonite or wollastonite than ortho-amphibolites.
• Geochemistry: Analyzing the rock’s chemical makeup, including all those trace elements. This can give us clues about where the original material came from.
• Geochronology: Dating the minerals to figure out when the metamorphism happened and, hopefully, how old the protolith is.
• Field Relationships: Looking at the big picture – what other rocks are nearby, and how are they all connected?

Why Bother? The Big Picture

Why go to all this trouble to figure out what an amphibolite used to be? Because it tells us so much about the Earth’s history! It helps us understand:

• Tectonic Setting: The protolith can hint at the tectonic environment where the rock formed – was it a mid-ocean ridge? An island arc? A continental rift?
• Metamorphic Conditions: The minerals in the amphibolite tell us about the temperature and pressure during metamorphism.
• Crustal Evolution: Studying these rocks helps us piece together how the Earth’s crust has changed over millions of years.

So, next time you see an amphibolite, remember it’s not just a rock. It’s a transformed survivor with a fascinating past. By understanding its protolith, we unlock a chapter in the Earth’s ongoing story.