It was shaping up to be a relatively quiet week for markets, until Friday delivered a cascade of market-moving headlines.

Fresh economic data showed the U.S. economy expanded at just a 1.4% annualized pace in the fourth quarter, a sharp slowdown from 4.4% in the prior quarter and well below expectations for 3% growth.

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At the same time, the Federal Reserve's preferred inflation gauge accelerated to a 3% annual rate in December, with the monthly increase of 0.4% marking the fastest pace since February.

President Donald Trump pointed to the prolonged government shutdown as a key driver of the weak growth print, arguing it shaved at least two percentage points off GDP.

But the real jolt came hours later, when the Supreme Court issued a landmark decision striking down the administration's tariff authority under the International Emergency Economic Powers Act (IEEPA).

The Court ruled that IEEPA does not authorize the president to impose tariffs, grounding its reasoning in separation of powers. "

“The Framers gave ‘Congress alone' the power to impose tariffs during peacetime," the Court wrote, adding "the whole ...